mesa: fix proxy textures becoming immutable and unusable

mesa: fix proxy textures becoming immutable and unusable

glTexStorage*() functions make textures immutable.  This carries on to
proxy textures.  Error checking in texture storage functions prevents
proxy textures from working after first time because internally, they
became immutable.

This commit makes the error checking ignore the immutability flag when
working with proxy textures.

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