mesa: fix multisampling proxy textures not being queryable

mesa: fix multisampling proxy textures not being queryable

The code that checks if some texture target is valid for
glGetTexLevelParameter*() was not programmed to check for multisampling
proxy textures.  This made it impossible(?) to use the proxy textures
for their intended purpose as glGetTexLevelParameter*() would just fail
on you.

Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: mesa-stable@lists.freedesktop.org