mesa: remove bogus compressed texture size checks

mesa: remove bogus compressed texture size checks

A compressed texture image size doesn't have to be a multiple of the
compressed block size (only sub-images do).  Fixes issues when building
compressed mipmaps because we often wind up with non-block-size images
for the higher mipmap levels.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=55445

Note: This is a candidate for the stable branches.

Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Sven Arvidsson <sa@whiz.se>