gallium/util: do not use undefined memory for texcoords

blitter_context_priv::vertices is initialized at start-up, but can (and
will) be overwritten by calls to the blitter since then. Since we always
copy from the LOD read by the the vertex-shader, and we risk blitting
from out-of-range LODs, which give incorrect results.

So let's just bite the bullet and not use the XY-codepath in the shared
code. The difference is just a few writes, but then we always update all
the read state as apropriate.

This fixes a case for me where the following two Khronos Confidental CTS
cases are run back to back:


Fixes: 22ed1ba0 ("gallium/u_blitter: use draw_rectangle for all blits except cubemaps")