intel: Don't call _mesa_get_format_bytes for MESA_FORMAT_NONE

intel: Don't call _mesa_get_format_bytes for MESA_FORMAT_NONE

When we don't intend to texture from or render to a __DRIimage we
use __DRI_IMAGE_FORMAT_NONE.  In that case, we just create the __DRIimage
to reference the underlying buffer, and will create usable __DRIimages
from it using createSubImage later.

If we try to use _mesa_get_format_bytes() on MESA_FORMAT_NONE in
a debug build, we hit an assertion, so let's not do that.