i965: Emit texture cache flushes on gen6 along with render cache flushes.

i965: Emit texture cache flushes on gen6 along with render cache flushes.

It turns out that internally the texture cache gets flushed in a
couple of cases, particularly around 2D operations mixed with 3D.  In
almost all cases one of those happens between rendering to an
FBO-attached texture and rendering from that texture.  However, as of
the next patch, glean tfbo (and the new fbo-flushing-2 test) would
manage to get stale texture values because one of those flushes didn't
occur.  The intention of this code was always to get the render cache
cleared and ready to be used from the sampler cache (and it does on <=
gen4), so this just catches gen5 up.

This patch was also tested to fix fbo-flushing on gen7.