drm/i915: Avoid moving from CPU domain during pwrite

We can avoid an early clflush when pwriting if we use the current CPU
write domain rather than moving the object to the GTT domain for the
purposes of the pwrite. This has the advantage of not flushing the
presumably hot data that we want to upload into the bo, and of ascribing
the clflush to the execution when profiling.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>