drm/i915: enable IPS for bpp <= 24

Art confirms that this should work fine.  Since most panels are 18bpp
with dithering from 24bpp, the existing code wouldn't be enabled in most
cases.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>