i965: Move buffer texture size calculation into a common helper function.

The buffer texture size calculations (should be easy enough, right?)
are repeated in three different places, each of them subtly broken in
a different way.  E.g. the image load/store path was never fixed to
clamp to MaxTextureBufferSize, and none of them are taking into
account the buffer offset correctly.  It's easier to fix it all in one
place.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106481


Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
(cherry picked from commit 156d2c6e)