drm/i915: Do not force non-caching copies for pwrite along shmem path

We don't always want to write into main memory with pwrite. The shmem
fast path in particular is used for memory that is cacheable - under
such circumstances forcing the cache eviction is undesirable. As we will
always flush the cache when targeting incoherent buffers, we can rely on
that second pass to apply the cache coherency rules and so benefit from
in-cache copies otherwise.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>