drm/i915: Fix kerneldoc warnings for i915_gem_userptr

drivers/gpu/drm/i915/i915_gem_userptr.c:761: warning: No description found for parameter 'dev'
drivers/gpu/drm/i915/i915_gem_userptr.c:761: warning: No description found for parameter 'data'
drivers/gpu/drm/i915/i915_gem_userptr.c:761: warning: No description found for parameter 'file'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208111328.32422-1-chris@chris-wilson.co.uk