drm/i915: Consolidate #ifdef CONFIG_INTEL_IOMMU

We depend on intel_iommu_gfx_mapped for various workarounds, but that is
only available under an #ifdef CONFIG_INTEL_IOMMU. Refactor all the
cut-and-paste ifdefs to a common routine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170525121612.2190-1-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>