drm/i915: Move whole object to CPU domain for coherent shmem access

drm/i915: Move whole object to CPU domain for coherent shmem access

If the object is coherent, we can simply update the cache domain on the
whole object rather than calculate the before/after clflushes. The
advantage is that we then get correct tracking of ellided flushes when
changing coherency later.

Testcase: igt/gem_pwrite_snooped
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170310000942.11661-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>