backend-drm: Cache drm_fb for each weston_buffer

When we first create a drm_fb from a weston_buffer, cache it and keep it
alive as long as the buffer lives. This allows us to reuse the gbm_bo
and kernel-side DRM framebuffer, rather than constantly creating and
destroying them at every repaint. The overhead of doing so (e.g. MMU
updates) can be significant on some platforms.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1198,6 +1198,7 @@ struct weston_buffer {
 	int32_t width, height;
 	uint32_t busy_count;
 	int y_inverted;
+	void *backend_private;
 };
 
 struct weston_buffer_reference {