i965/fs: Reduce the interference between payload regs and virtual GRFs.

i965/fs: Reduce the interference between payload regs and virtual GRFs.

Improves performance of the Lightsmark penumbra shadows scene by 15.7% +/-
1.0% (n=15), by eliminating register spilling. (tested by smashing the list of
scenes to have all other scenes have 0 duration -- includes additional
rendering of scene description text that normally doesn't appear in that
scene)

v2: Allow allocation of all but g0/g1 of the payload.
v3: Pull count_to_loop_end() out to a helper function.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v2, recommended v3)