To align with the ilk WM code and because it makes sense to test against
the upper bounds as soon as possible on variables that are bigger than
the number of bits in the register, let's move the maximum checks from
skl_compute_wm_results() to skl_compute_plane_wm().

v2: Leave the result values to 0 when overflowing the limits (Ville)
    Use 32 bits intermediate variables (Damien)

Instead of using the 16 and 8 bits space we have in the result
structure, use 32 bits local variables until we're sure they fit into
the constraints.
Suggested-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>