drm/i915: Use consistent mappings for OpRegion between ACPI and i915

The opregion is a shared memory region between ACPI and the graphics
driver. As the ACPI mapping has been changed to cachable in commit
6d5bbf00

, mapping the intel opregion
non-cachable now fails. As no bus-master hardware is involved in the
opregion, cachable map should do no harm.

Tested on a Fujitsu Lifebook P8010.

Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
[ickle: convert to acpi_os_ioremap for consistency]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>