i965: Rework Sandybridge 3DSTATE_VIEWPORT_STATE_POINTERS.

i965: Rework Sandybridge 3DSTATE_VIEWPORT_STATE_POINTERS.

On Gen7+ we emit 3DSTATE_VIEWPORT_STATE_POINTERS_{SF_CL,CC} when
emitting a new viewport.

This patch makes us take the same approach on Sandybridge - but because
we have a combined command, we just set the appropriate "change" bits.
This eliminates an atom, some dirty flagging, and some brw->*.vp_offset
writes.  It does mean we'll emit two 3DSTATE_VIEWPORT_STATE_POINTERS
instead of one if both change, but that's probably fine.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>