drm/format-helper: Print warning on missing format conversion

Not all possible format conversions are supported yet. Print a
warning on unsupported combinations.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425075939.30450-2-tzimmermann@suse.de

drivers/gpu/drm/drm_format_helper.c

@@ -590,6 +590,9 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
 		}
 	}
 
+	drm_warn_once(fb->dev, "No conversion helper from %p4cc to %p4cc found.\n",
+		      &fb_format, &dst_format);
+
 	return -EINVAL;
 }
 EXPORT_SYMBOL(drm_fb_blit_toio);