freedreno/a3xx: deal with optimized tex instructions

Keep track of whether we actually have any sam instructions in the
resulting shader, rather than using TGSI SAMP declarations.  If the sam
instruction is optimized out, because the result is not used, we don't
want to emit texture state, etc.  In fact emitting sampler state and/or
setting PIXLODENABLE bit when there are no texture fetches seems to
cause lockup.

In theory this should never happen for a "normal" shader, unless the
state tracker is wonky.  But it is a very real possibility for binning
pass shaders.

Signed-off-by: Rob Clark <robclark@freedesktop.org>