drm/i915: Remove IS_ULT()

As stated in the few previous commits, IS_ULT/ULX() is better
per-platform as it has different consequences depending on the platform.

We now can get rid of it.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>