i965/fs: Factor out calculation of the block of MRFs reserved for spilling.
i965/fs: Factor out calculation of the block of MRFs reserved for spilling.

And as we're at it fix the calculation to allocate a larger block of
registers for 32-wide dispatch.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler