i965: CS writes/reads should use I915_GEM_INSTRUCTION

i965: CS writes/reads should use I915_GEM_INSTRUCTION

Otherwise the gen6 w/a in the kernel won't kick in and the write will
land nowhere.

Inspired by a patch Ken pointed me at which had the same issue (but
isn't yet merged and also for a gen7+ feature). An audit of the entire
driver didn't reveal any other case than the one in in the write_reg
helper used by the gen6 queryobj code.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Xinkai Chen <yeled.nova@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: "9.2" <mesa-stable@lists.freedesktop.org>