drm/i915: [sparse] __iomem fixes for overlay

With the exception of a forced cast for phys_obj stuff (a problem in
other patches as well) all of these are fairly simple __iomem compliance
fixes.

As with other patches, yank/paste errors may exist.

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
[danvet: Added comment to explain the __iomem cast.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>