i965/fs: Make emit_spill/unspill static functions taking builder as argument.

i965/fs: Make emit_spill/unspill static functions taking builder as argument.

This seems cleaner than exposing an implementation detail of
brw_fs_reg_allocate.cpp to the world, and will give the caller control
over the instruction execution flags (e.g. force_writemask_all) that
are applied to the scratch read and write instructions.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>