drm/amd/display: Check writeback connectors in create_validate_stream_for_sink

[WHY & HOW]
This is to check connector type to avoid
unhandled null pointer for writeback connectors.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Fixes: 60e034f2 ("drm/amd/display: Revert "drm/amd/display: Use drm_connector in create_validate_stream_for_sink"")
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>