i965: Combine URB code emission into a single group.

i965: Combine URB code emission into a single group.

All four URB packets need to be programmed together in order for the GPU
state to be valid.  Putting them in separate BEGIN..ADVANCE blocks is
risky: if we're nearing the end of a batch, the batch could be flushed
inbetween two of the commands, causing the URB programming to be split
into two batchbuffers.

This -might- be okay with hardware contexts, but it offers no advantages
over keeping them together, and has a potential for hangs.

Putting them into a single BEGIN..ADVANCE block ensures they'll be kept
in the same batch, which seems wise.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>