drm/vgem: Move get_pages to gem_create

vgem doesn't have a shrinker or anything like that and drops backing
storage only at object_free time. There's no use in trying to be
clever and allocating backing storage delayed, it only causes trouble
by requiring locking.

Instead grab pages when we allocate the object right away.

v2: Fix compiling.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-12-git-send-email-daniel.vetter@ffwll.ch