drm/i915/skl: Move all the WM compute functions in one place

The DDB allocation code managed to split in two the compute functions.

Bring back skl_compute_transition_wm() and skl_compute_linetime_wm()
with their little friends.

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>