drm/i915: mark a newly-created GEM object dirty when filled with data

When creating a new (pageable) GEM object and filling it with data, we
must mark it as 'dirty', i.e. backing store is out-of-date w.r.t. the
newly-written content. This ensures that if the object is evicted under
memory pressure, its pages in the pagecache will be written to backing
store rather than discarded.

Based on an original version by Alex Dai.

Signed-off-by: Alex Dai <yu.dai@intel.com>
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1449773486-30822-3-git-send-email-david.s.gordon@intel.com


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