drm/i915: Make GSM void

The iomapping of the register region has historically been a uint32_t
for the obvious reason that our PTE size was always 4b. In the future
however, we cannot make this assumption.

By making the type void, it makes the upcoming pointer math we will do
much easier, and hopefully gives the compiler opportunities to warn us
when we do stupid things.

v2: Cast to __iomem, caught by Ville

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Fixup __iomem issue for real.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>