Revert "mesa: simplify _mesa_is_image_unit_valid for buffers"

This reverts commit c0ed52f6.  It was
preventing the image format validation from being done on buffer
textures, which is required to ensure that the application doesn't
attempt to bind a buffer texture with an internal format incompatible
with the image unit format (e.g. of different texel size), which is
not allowed by the spec (it's not allowed for *any* texture target,
whether or not there is spec wording restricting this behavior
specifically for buffer textures) and will cause the driver to
calculate texel bounds incorrectly and potentially crash instead of
the expected behavior.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106465


Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
(cherry picked from commit 5a681478)