backend-drm: Retrieve reason if dmabuf import failed

As we could have situations where dmabuf import failed when attempting
to figure it the framebuffer is scanout-capable, make sure we also have
a way to store that information. Otherwise, we could end up
NULL-dereferencing, as we don't provide a valid storage for it.

Further more, with this, we also print out the reason why it failed, to
aid in further debugging.

Observed on platforms where GBM_BO_HANDLE failed + in combination w/
direct-display proto extension.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/backend-drm/fb.c

@@ -454,14 +454,15 @@ drm_can_scanout_dmabuf(struct weston_compositor *ec,
 	struct drm_fb *fb;
 	struct drm_backend *b = to_drm_backend(ec);
 	bool ret = false;
+	uint32_t try_reason = 0x0;
 
-	fb = drm_fb_get_from_dmabuf(dmabuf, b, true, NULL);
+	fb = drm_fb_get_from_dmabuf(dmabuf, b, true, &try_reason);
 	if (fb)
 		ret = true;
 
 	drm_fb_unref(fb);
-	drm_debug(b, "[dmabuf] dmabuf %p, import test %s\n", dmabuf,
-		      ret ? "succeeded" : "failed");
+	drm_debug(b, "[dmabuf] dmabuf %p, import test %s, with reason 0x%x\n", dmabuf,
+		      ret ? "succeeded" : "failed", try_reason);
 	return ret;
 }