drm/i915/skl: Rework when the transition WMs are computed

The transition WMs code was doing a shortcut and the values were copied
from the WM0 ones at compute_wm_results() time. Going forward, we want
to compute them like the other WMs and resolve their final register
values in the same way as well.

This patch does just that and isolate the transtion WM compute code in
skl_compute_transition_wm() while skl_compute_wm_results() takes care of
the register values.

We also take the opportunity to disable the transition WMs for now.
We've noticed underruns and they seem to be the culprit.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>