drm/amd/display: Check null pointers before using them

[WHAT & HOW]
dc_link is null checked previously in the same function, indicating it
might be null as reported by Coverity.

This fixes 1 FORWARD_NULL issue reported by Coverity.
Signed-off-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -12083,6 +12083,7 @@ void amdgpu_dm_update_freesync_caps(struct drm_connector *connector,
 		}
 	}
 
+	if (amdgpu_dm_connector->dc_link)
 		as_type = dm_get_adaptive_sync_support_type(amdgpu_dm_connector->dc_link);
 
 	if (as_type == FREESYNC_TYPE_PCON_IN_WHITELIST) {