freedreno/a6xx: more bcolor fixes

Non-zero offset wasn't working, which breaks a bunch of
dEQP-GLES31.functional.texture.border_clamp.formats.* when doing sharded
deqp runs (because order of tests changes, resulting in different
texture state bound.. deqp doesn't really clean up it's gl state between
tests very well)

Previously, if additional textures were bound, due to using too small of
a bcolor_entry size, the last 32bytes of the bcolor_entry would be
overwritten.

Signed-off-by: Rob Clark <robdclark@gmail.com>