drm/i915: Check for get_pages instead of shmem (filp)

This behavior of checking for a shmem backed GEM object was introduced here:
commit 4c914c0c
Author: Brad Volkin <bradley.d.volkin@intel.com>
Date:   Tue Feb 18 10:15:45 2014 -0800

    drm/i915: Refactor shmem pread setup

It is possible for an object to not be a shmem backed GEM object (for example
userptr objects). An example of how we hit this failure can be found through
copy_batch() in the command parser because we allocate a userptr object for the
batch which contains privileged instructions. Userptr calls
drm_gem_private_object_init() which explicitly sets the filp to none.

NOTE: I manually retyped this from a test machine. So I haven't even compiled
this exact patch.

v2: Use same logic as from a2a4f916c2f (Kristian, Dave Gordon)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com> (v1)
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455047053-2644-1-git-send-email-benjamin.widawsky@intel.com