drm/i915/skl: Updated watermark programming

Recent BSpect updates have changed the watermark calculation to avoid
display flickering in some cases.

v2: Fix check against DDB allocation and tidy the code a bit. (Damien Lespiau)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>