drm/amd/display: Return drm_connector from find_first_crtc_matching_connector

[WHY]
We will be dealing with two types of connector: amdgpu_dm_connector
and drm_writeback_connector.

[HOW]
We want to find both and then cast to the appriopriate type afterwards.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>