drm/i915: Refactor shmem pread setup

The command parser is going to need the same synchronization and
setup logic, so factor it out for reuse.

v2: Add a check that the object is backed by shmem

Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>