drm: check for PRIME support

PRIME support for buffer sharing has become mandatory since the renderer
rewrite. Make sure we check for the appropriate capabilities in backend,
allocator and renderer.

See also #2819.

render/gbm_allocator.c

@@ -160,6 +160,13 @@ static struct wlr_gbm_allocator *get_gbm_alloc_from_alloc(
 }
 
 struct wlr_gbm_allocator *wlr_gbm_allocator_create(int fd) {
+	uint64_t cap;
+	if (drmGetCap(fd, DRM_CAP_PRIME, &cap) ||
+			!(cap & DRM_PRIME_CAP_EXPORT)) {
+		wlr_log(WLR_ERROR, "PRIME export not supported");
+		return NULL;
+	}
+
 	struct wlr_gbm_allocator *alloc = calloc(1, sizeof(*alloc));
 	if (alloc == NULL) {
 		return NULL;