drm/i915: Make a single set-to-cpu-domain path and use it wherever needed.

This fixes several domain management bugs, including potential lack of cache
invalidation for pread, potential failure to wait for set_domain(CPU, 0),
and more, along with producing more intelligible code.

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