Revert "drm/amd/display: Return drm_connector from find_first_crtc_matching_connector"

This reverts commit ca2b37a1.

[WHY & HOW]
The writeback series cause a regression in thunderbolt display.

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