mesa/meta: check for signed/unsigned int conversion for pbo getteximage

mesa/meta: check for signed/unsigned int conversion for pbo getteximage

When doing GetTexSubImage using a PBO, we should check if it involves
a signed/unsigned conversion and bail if it does, just like in the
other cases.

This fixes:
GL33-CTS.gtf32.GL3Tests.packed_pixels.packed_pixels_pbo
on Haswell at least.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95324
Reviewed-by: Matt Turer <mattst88@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>