drm/amd/display: Use drm_connector in create_validate_stream_for_sink

[WHAT]
Again, we need to use this function for writeback connectors,
which are not of type amdgpu_dm_connector. Use the common base
drm_connector instead.

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>