i915: switch from acpi_os_ioremap to memremap

i915 expects the OpRegion to be cached (i.e. not __iomem), so explicitly
map it with memremap rather than the implied cache setting of
acpi_os_ioremap().

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>