mesa: Update image unit state when glBindImageTexture is called with texture=0.

mesa: Update image unit state when glBindImageTexture is called with texture=0.

There's no indication in the spec that the image unit state other than the
bound texture object shouldn't be updated when glBindImageTexture() is called
passing the zero texture as argument.  It's very unlikely that any application
would ever have relied on this, but it's easy to get right, and it fixes the
"state" ARB_shader_image_load_store piglit test.

Reviewed-by: Matt Turner <mattst88@gmail.com>