drm/amd/display: Skip writeback connector when we get amdgpu_dm_connector

[WHY]
Writeback connectors are based on a different object:
drm_writeback_connector, and are therefore different from
amdgpu_dm_connector. We need to be careful to ensure code
designed for amdgpu_dm_connector doesn't inadvertently try
to operate on a drm_writeback_connector.

[HOW]
Skip them when connector type is DRM_MODE_CONNECTOR_WRITEBACK.

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>