drm/i915: Rename object_set_domain to object_set_to_gpu_domain

Now that the CPU and GTT domain operations are isolated to their own
functions, the previously general-purpose set_domain function is now used
only to set GPU domains. It also has no failure cases, which is important as
this eliminates any possible interruption of the computation of new object
domains and subsequent emmission of the flushing instructions into the ring.

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