i965: Replace #include "intel_context.h" with brw_context.h.

i965: Replace #include "intel_context.h" with brw_context.h.

brw_context.h includes intel_context.h, but additionally makes the
brw_context structure available.  Switching this allows us to start
using brw_context in more places.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Paul Berry <stereotype441@gmail.com>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>