vo_opengl: vaapi: document DRM fourcc upstream defines

Add the upstream symbolic names as comments. Normally, these should be
defined in libdrm's drm_fourcc.h header. But DRM_FORMAT_R8 and
DRM_FORMAT_GR88 are not defined anywhere, except in the kernel userland
headers of Linux 4.3 (!). We don't want mpv to depend on bleeding-edge
Linux kernel headers, so this will have to do.

Also, just for completeness, add fourccs for the 3 and 4 channel
formats. I didn't manage to test them, though.

video/out/opengl/hwdec_vaegl.c

@@ -200,9 +200,10 @@ static int map_image(struct gl_hwdec *hw, struct mp_image *hw_image,
     mp_image_setfmt(&layout, mpfmt);
 
     // (it would be nice if we could use EGL_IMAGE_INTERNAL_FORMAT_EXT)
-    int drm_fmts[4] = {MP_FOURCC('R', '8', ' ', ' '),
-                       MP_FOURCC('G', 'R', '8', '8'),
-                       0, 0};
+    int drm_fmts[4] = {MP_FOURCC('R', '8', ' ', ' '),   // DRM_FORMAT_R8
+                       MP_FOURCC('G', 'R', '8', '8'),   // DRM_FORMAT_GR88
+                       MP_FOURCC('R', 'G', '2', '4'),   // DRM_FORMAT_RGB888
+                       MP_FOURCC('R', 'A', '2', '4')};  // DRM_FORMAT_RGBA8888
 
     for (int n = 0; n < layout.num_planes; n++) {
         int attribs[20] = {EGL_NONE};