drm/i915: [sparse] __iomem fixes for gem

As with one of the earlier patches in the series, we're forced to cast
for copy_[to|from]_user. Again because of the nature of the GEN x86
exclusivity, this should be safe.

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
[danvet: Added some bikeshed.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>