i965: Define and use REG_MASK macro to make masked MMIO writes slightly more readable.

i965: Define and use REG_MASK macro to make masked MMIO writes slightly more readable.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg  <krh@bitplanet.net>