drm/amd/display: Add null checker before passing variables

Checks null pointer before passing variables to functions.

This fixes 3 NULL_RETURNS issues reported by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2875,6 +2875,7 @@ static int dm_suspend(void *handle)
 
 		dm->cached_dc_state = dc_state_create_copy(dm->dc->current_state);
 
+		if (dm->cached_dc_state)
 			dm_gpureset_toggle_interrupts(adev, dm->cached_dc_state, false);
 
 		amdgpu_dm_commit_zero_streams(dm->dc);
@@ -7102,6 +7103,7 @@ static void create_eml_sink(struct amdgpu_dm_connector *aconnector)
 		aconnector->dc_sink = aconnector->dc_link->local_sink ?
 		aconnector->dc_link->local_sink :
 		aconnector->dc_em_sink;
+		if (aconnector->dc_sink)
 			dc_sink_retain(aconnector->dc_sink);
 	}
 }
@@ -7929,6 +7931,7 @@ static int amdgpu_dm_connector_get_modes(struct drm_connector *connector)
 				drm_add_modes_noedid(connector, 1920, 1080);
 	} else {
 		amdgpu_dm_connector_ddc_get_modes(connector, edid);
+		if (encoder)
 			amdgpu_dm_connector_add_common_modes(encoder, connector);
 		amdgpu_dm_connector_add_freesync_modes(connector, edid);
 	}