glamor_egl: Don't initialize on llvmpipe

Mesa started supporting GL_OES_EGL_image on llvmpipe in 17.3, after this
commit:

    commit bbdeddd5fd0b797e1e281f058338b3da4d98029d
    Author: Gurchetan Singh <gurchetansingh@chromium.org>
    Date:   Tue Aug 1 14:49:33 2017 -0700

        st/dri: add drisw image extension

That's pretty cool, but it means glamor now thinks it can initialize on
llvmpipe. This is almost certainly not what anyone wants, as glamor on
llvmpipe is pretty much uniformly slower than fb.

This fixes both Xorg and Xwayland to refuse glamor in such a setup.
Xephyr is left alone, both because glamor is not the default there and
because Xephyr+glamor+llvmpipe is one of the easier ways to get xts to
exercise glamor.

The (very small) downside of this change is that you lose DRI3 support.
This wouldn't have helped you very much (since an lp glamor blit is
slower than a pixman blit), but it would eliminate the PutImage overhead
for llvmpipe's glXSwapBuffers. A future change should add DRI3 support
for the fb-only case.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>