intel/compiler: Prevent warnings in the following patch

The next patch replaces an unsigned bitfield with a plain unsigned,
which triggers gcc to begin warning on signed/unsigned comparisons.

Keeping this patch separate from the actual move allows bisectablity and
generates no additional warnings temporarily.

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

src/intel/compiler/brw_fs_reg_allocate.cpp

@@ -912,7 +912,7 @@ fs_visitor::choose_spill_reg(struct ra_graph *g)
 }
 
 void
-fs_visitor::spill_reg(int spill_reg)
+fs_visitor::spill_reg(unsigned spill_reg)
 {
    int size = alloc.sizes[spill_reg];
    unsigned int spill_offset = last_scratch;