mesa: properly return GetTexLevelParameter queries for buffer textures

mesa: properly return GetTexLevelParameter queries for buffer textures

This fixes all failures with dEQP tests in this area. While
ARB_texture_buffer_object explicitly says that GetTexLevelParameter & co
should not be supported, GL 3.1 reverses this decision and allows all of
these queries there.

Conversely, there is no text that forbids the buffer-specific queries
from being used with non-buffer images.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Brian Paul <brianp@vmware.com>