drm/amd/display: Check NULL connector before it is used

drm/amd/display: Check NULL connector before it is used

[Why & How]
amdgpu_dm_find_first_crtc_matching_connector can return NULL.
It is necessary to the returned connector before passing it
drm_atomic_get_new_connector_state which always assumes connector is
not NULL.

Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>