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: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>