drm/amd/display: guard for virtual calling destroy_link_encoders

[Why]:
On power down, virtual dal may try to delete link_encoders by
referencing uninitialized res_pool.

[How]:
Added guard against empty res_pool.
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Martin Leung <martin.leung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc.c

@@ -347,10 +347,16 @@ static bool create_link_encoders(struct dc *dc)
  */
 static void destroy_link_encoders(struct dc *dc)
 {
-	unsigned int num_usb4_dpia = dc->res_pool->res_cap->num_usb4_dpia;
-	unsigned int num_dig_link_enc = dc->res_pool->res_cap->num_dig_link_enc;
+	unsigned int num_usb4_dpia;
+	unsigned int num_dig_link_enc;
 	int i;
 
+	if (!dc->res_pool)
+		return;
+
+	num_usb4_dpia = dc->res_pool->res_cap->num_usb4_dpia;
+	num_dig_link_enc = dc->res_pool->res_cap->num_dig_link_enc;
+
 	/* A platform without USB4 DPIA endpoints has a fixed mapping between DIG
 	 * link encoders and physical display endpoints and does not require
 	 * additional link encoder objects.