i965/fs: Replace usage is_tex() with regs_written() checks.

i965/fs: Replace usage is_tex() with regs_written() checks.

In these places, we care about any sort of send that hits more than one reg,
not just textures.  We don't yet have anything else returning more than one
reg, so there's no change.

v2: Use mlen instead of is_tex() for the is-it-a-send check.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>