mesa: fix format selection for meta CopyTexSubImage()

mesa: fix format selection for meta CopyTexSubImage()

When we do a glReadPixels into the temporary buffer, we don't want to
use GL_LUMINANCE, GL_LUMINANCE_ALPHA or GL_INTENSITY since they will
compute L=R+G+B which is not what we want.

This bug has existed all along but was only exposed by the elimination
of the driver hook for glCopyTexImage() in
5874890c26f434f54e9218b83fae4eb8175c24e9.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=39604
Tested-by: Ian Romanick <ian.d.romanick@intel.com>