i965: Assign PS kernel start pointers when we decide which kernels to use

i965: Assign PS kernel start pointers when we decide which kernels to use

Right now we decide which kernels to use and the GRF start offsets in
one place and emit the kernel pointers later.  The logic of how to map
8, 16 and 32 kernels to kernel start pointers follows the same logic as which
GRF start offsets to use, so lets figure out these two things in one place.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>