mesa: fix number of mipmaps calculation for proxy textures

mesa: fix number of mipmaps calculation for proxy textures

The function _mesa_get_tex_max_num_levels() is supposed to calculate
the number of mipmap levels but it was not written to handle proxy
textures, at best returning a maximum of 1 mipmap level. Because of
this, at least glTexStorage*() calls would incorrectly fail when used
with proxy textures with more than one mipmap level.

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