intel/fs: Mark LINTERP opcode as writing accumulator on platforms without PLN

When we don't have PLN (gen4 and gen11+), we implement LINTERP as either
LINE+MAC or a pair of MADs.  In both cases, the accumulator is written
by the first of the two instructions and read by the second.  Even
though the accumulator value isn't actually ever used from a logical
instruction perspective, it is trashed so we need to make the scheduler
aware.  Otherwise, the scheduler could end up re-ordering instructions
and putting a LINTERP between another an instruction which writes the
accumulator and another which tries to use that result.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 566e6abd)
Rebased version provided by Jason