drm/i915: Allow use of get_dma_address for stolen backed objects

i915_gem_object_get_dma_address function is used to retrieve the dma address
of a particular page so as to map it in a given GTT entry for CPU access.
This function would be used for stolen backed objects also for tasks like
pwrite,  clearing of the pages etc. So the obj->get_page.sg needs to be
initialized for the stolen objects also.

Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450765253-32104-2-git-send-email-ankitprasad.r.sharma@intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>