drm/amd/display: Skip on writeback when it's not applicable

[WHY]
dynamic memory safety error detector (KASAN) catches and generates error
messages "BUG: KASAN: slab-out-of-bounds" as writeback connector does not
support certain features which are not initialized.

[HOW]
Skip them when connector type is DRM_MODE_CONNECTOR_WRITEBACK.

Link: drm/amd#3199


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>