drm/i915: silence io mapping/unmapping sparse warnings on different address spaces

drivers/gpu/drm/i915/i915_gem_execbuffer.c:432:52: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/i915/i915_gem_execbuffer.c:432:52:    expected void [noderef] <asn:2>*vaddr
drivers/gpu/drm/i915/i915_gem_execbuffer.c:432:52:    got void *
drivers/gpu/drm/i915/i915_gem_execbuffer.c:477:15: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/i915/i915_gem_execbuffer.c:477:15:    expected void *vaddr
drivers/gpu/drm/i915/i915_gem_execbuffer.c:477:15:    got void [noderef] <asn:2>*

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475574853-4178-2-git-send-email-jani.nikula@intel.com