drm/amd/display: Reject cursor plane on DCE when scaled differently than primary

drm/amd/display: Reject cursor plane on DCE when scaled differently than primary

Currently DCE doesn't support the overlay cursor, so the
dm_crtc_get_cursor_mode() function returns DM_CURSOR_NATIVE_MODE
unconditionally. The outcome is that it doesn't check for the
conditions that would necessitate the overlay cursor, meaning
that it doesn't reject cases where the native cursor mode isn't
supported on DCE.

Remove the early return from dm_crtc_get_cursor_mode() for
DCE and instead let it perform the necessary checks and
return DM_CURSOR_OVERLAY_MODE. Add a later check that rejects
when DM_CURSOR_OVERLAY_MODE would be used with DCE.

Fixes: 1b04dcca4fb1 ("drm/amd/display: Introduce overlay cursor mode")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4600
Suggested-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>