i965/fs: Move payload register info from brw_wm_compile to fs_visitor.

i965/fs: Move payload register info from brw_wm_compile to fs_visitor.

This data is created by fs_visitor and only used when emitting code,
so keeping it in fs_visitor makes sense.  I decided it would be
reasonable to group these all together in a struct, since they're
highly related.

v2: s/nr_payload_regs/payload.num_regs/ in some comments (chrisf).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>