softpipe: Fix softpipe_is_texture_referenced.

softpipe: Fix softpipe_is_texture_referenced.

Render results are only visible when the render cache is flushed.
softpipe_is_texture_referenced must reflect that or transfers to/from the
textures bound in the framebuffer won't be proceeded of the necessary
flush, causing transfer data to be outdated/clobbered.

This fixes conform drawpix test with softpipe.