drm/i915/selftests: Markup __iomem for igt_gem_coherency

Silence sparse warnings by using __iomem markup and io accessors.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114191842.19063-1-chris@chris-wilson.co.uk