dri_util: Update internal_format to GL_RGB8 for MESA_FORMAT_B8G8R8X8_UNORM

Port the change done for RGBX8888 in 02a1f953.

If XR24 images are considered to be VIEW_CLASS_24_BITS-compatible, it's reasonable to assume that XB24 images would be as well.

Fixes: bf576772 ("dri_util: add driImageFormatToSizedInternalGLFormat function")