vc4: Fix subimage accesses to LT textures.

vc4: Fix subimage accesses to LT textures.

This code started out like the T case, iterating over utile offsets, but I
had partially switched it to iterating over pixel offsets.  I hadn't
caught this before because it's unusual to do piecemeal uploads to small
textures.

Fixes bad text rendering in QT5 apps, which use a 256x16 glyph cache.
Also fixes 6 piglit tests related to glTexSubImage() and
glGetTexSubImage().

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>