drm/i915: Move the execbuffer domain computations together

This eliminates the dev_set_domain function and just in-lines it
where its used, with the goal of moving the manipulation and use of
invalidate_domains and flush_domains closer together. This also
avoids calling add_request unless some domain has been flushed.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>