drm/i915: Fix the offset issue for the stolen GEM objects

drm/i915: Fix the offset issue for the stolen GEM objects

The 'offset' field of the 'scatterlist' structure was wrongly
programmed with the offset value from the base of stolen area,
whereas this field indicates the offset from where the interested
data starts within the first PAGE pointed to by 'scattterlist'
structure. As a result when a new GEM object allocated from stolen
area is mapped to GTT, it could lead to an overwrite of GTT entries
as the page count calculation will go wrong, refer the function
'sg_page_count'.

v2: Modified the commit message. (Chris)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71908
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69104
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>