drm/amd/display: Remove Assert from dcn10_get_dig_frontend

[Why]
In some cases, this function is called when DIG BE is not
connected to DIG FE, in which case a value of zero isn't
invalid and assert should not be hit.

[How]
Remove assert and handle ENGINE_ID_UNKNOWN result in calling
function.
Signed-off-by: NEric Bernstein <eric.bernstein@amd.com>
Acked-by: NBindu Ramamurthy <bindu.r@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_link_encoder.c

@@ -480,7 +480,6 @@ unsigned int dcn10_get_dig_frontend(struct link_encoder *enc)
 		break;
 	default:
 		// invalid source select DIG
-		ASSERT(false);
 		result = ENGINE_ID_UNKNOWN;
 	}
 

drivers/gpu/drm/amd/display/dc/dcn30/dcn30_hwseq.c

@@ -539,6 +539,8 @@ void dcn30_init_hw(struct dc *dc)
 
 					fe = dc->links[i]->link_enc->funcs->get_dig_frontend(
 										dc->links[i]->link_enc);
+					if (fe == ENGINE_ID_UNKNOWN)
+						continue;
 
 					for (j = 0; j < dc->res_pool->stream_enc_count; j++) {
 						if (fe == dc->res_pool->stream_enc[j]->id) {