drm/amdgpu/display: Add amdgpu_display.c documentation

drm/amdgpu/display: Add amdgpu_display.c documentation

Add kernel-doc references for amdgpu_display.c to the display manager
documentation. This pulls in documentation for display core functions
like the hotplug work handler, and the adaptive backlight modulation
property.

The :internal: directive automatically includes all function documentation,
while the explicit :doc: directive captures the property documentation that
:internal: doesn't pull in.

Fixes: 1454642960b0 ("drm/amd: Re-introduce property to control adaptive backlight modulation")
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Link: https://patch.msgid.link/20260701170004.465737-1-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>