intel/fs: Initialize fs_visitor::grf_used on construction.

This should shut up some Valgrind errors during pre-regalloc
scheduling.  The errors were harmless since they could only have led
to the estimation of the bank conflict penalty of an instruction
pre-regalloc, which is inaccurate at that point of the program
compilation, but no less accurate than the intended "return 0"
fall-back path.  The scheduling pass is normally re-run after regalloc
with a well-defined grf_used value and accurate bank conflict
information.

Fixes: acf98ff933 "intel/fs: Teach instruction scheduler about GRF bank conflict cycles."
Reported-by: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/intel/compiler/brw_fs_visitor.cpp

@@ -898,6 +898,7 @@ fs_visitor::init()
 
    this->promoted_constants = 0,
 
+   this->grf_used = 0;
    this->spilled_any_registers = false;
 }