drm/amd/display: Fix dc_link NULL handling in HPD init

drm/amd/display: Fix dc_link NULL handling in HPD init

amdgpu_dm_hpd_init() may see connectors without a valid dc_link.

The code already checks dc_link for the polling decision, but later
unconditionally dereferences it when setting up HPD interrupts.

Assign dc_link early and skip connectors where it is NULL.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_irq.c:940 amdgpu_dm_hpd_init()
error: we previously assumed 'dc_link' could be null (see line 931)

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_irq.c
    923                 /*
    924                  * Analog connectors may be hot-plugged unlike other connector
    925                  * types that don't support HPD. Only poll analog connectors.
    926                  */
    927                 use_polling |=
    928                         amdgpu_dm_connector->dc_link &&
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The patch adds this NULL check but hopefully it can be removed

    929                         dc_connector_supports_analog(amdgpu_dm_connector->dc_link->link_id.id);
    930
    931                 dc_link = amdgpu_dm_connector->dc_link;

dc_link assigned here.

    932
    933                 /*
    934                  * Get a base driver irq reference for hpd ints for the lifetime
    935                  * of dm. Note that only hpd interrupt types are registered with
    936                  * base driver; hpd_rx types aren't. IOW, amdgpu_irq_get/put on
    937                  * hpd_rx isn't available. DM currently controls hpd_rx
    938                  * explicitly with dc_interrupt_set()
    939                  */
--> 940                 if (dc_link->irq_source_hpd != DC_IRQ_SOURCE_INVALID) {
                            ^^^^^^^^^^^^^^^^^^^^^^^ If it's NULL then we are trouble because we dereference it here.

    941                         irq_type = dc_link->irq_source_hpd - DC_IRQ_SOURCE_HPD1;
    942                         /*
    943                          * TODO: There's a mismatch between mode_info.num_hpd
    944                          * and what bios reports as the # of connectors with hpd

Fixes: 4562236b3bc0 ("drm/amd/dc: Add dc display driver (v2)")
Cc: Timur Kristóf <timur.kristof@gmail.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Mario Limonciello <superm1@kernel.org>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: ChiaHsuan Chung <chiahsuan.chung@amd.com>
Cc: Roman Li <roman.li@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>