drm,io-mapping: Specify slot to use for atomic mappings

This is required should we ever attempt to use an io-mapping where
KM_USER0 is verboten, such as inside an IRQ context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>