drm/i915: Silence sparse complaints over insufficient bitfield int types.

drivers/gpu/drm/i915/i915_drv.h|676 col 19| warning: dubious bitfield without explicit `signed' or `unsigned'
drivers/gpu/drm/i915/i915_drv.h|712 col 19| warning: dubious bitfield without explicit `signed' or `unsigned'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>