vc4: Avoid loading from the texture during non-utile-aligned glTexImage().

vc4: Avoid loading from the texture during non-utile-aligned glTexImage().

Previously, the plan was "if the width/height we have to load/store isn't
the size the user is planning on writing, then we need to load the old
contents out beforehand to prevent writing back undefined".

However, when we're doing glTexImage() we often end up aligning the
width/height into the padding of the texture, and we don't actually
need to read out that padding.

Improves x11perf -aatrapezoid100 performance from ~460/sec to
~700/sec.