drm/i915: io unmap functions want __iomem

drm/i915: io unmap functions want __iomem

Don't cast away the __iomem from the io_mapping functions so that
sparse won't be so unhappy when we pass the pointer to the unmap
functions. Instead let's move the cast to where we actually use the
pointer.

Fixes the following sparse warnings:
i915_gem.c:1022:33: warning: incorrect type in argument 1 (different address spaces)
i915_gem.c:1022:33:    expected void [noderef] <asn:2>*vaddr
i915_gem.c:1022:33:    got void *[assigned] vaddr
i915_gem.c:1027:34: warning: incorrect type in argument 1 (different address spaces)
i915_gem.c:1027:34:    expected void [noderef] <asn:2>*vaddr
i915_gem.c:1027:34:    got void *[assigned] vaddr
i915_gem.c:1199:33: warning: incorrect type in argument 1 (different address spaces)
i915_gem.c:1199:33:    expected void [noderef] <asn:2>*vaddr
i915_gem.c:1199:33:    got void *[assigned] vaddr
i915_gem.c:1204:34: warning: incorrect type in argument 1 (different address spaces)
i915_gem.c:1204:34:    expected void [noderef] <asn:2>*vaddr
i915_gem.c:1204:34:    got void *[assigned] vaddr

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901171252.31025-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>