drm/i915: fall through pwrite_gtt_slow to the shmem slow path

The gtt_pwrite slowpath grabs the userspace memory with
get_user_pages. This will not work for non-page backed memory, like a
gtt mmapped gem object. Hence fall throuh to the shmem paths if we hit
-EFAULT in the gtt paths.

Now the shmem paths have exactly the same problem, but this way we
only need to rearrange the code in one write path.

v2: v1 accidentaly falls back to shmem pwrite for phys objects. Fixed.

v3: Make the codeflow around phys_pwrite cleara as suggested by Chris
Wilson.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>