drm/i915: Reinstate an early latency==0 check for skl+

drm/i915: Reinstate an early latency==0 check for skl+

I thought we could remove all the early latency==0 checks
and rely on skl_wm_method{1,2}() checking for it. But
skl_compute_plane_wm() applies a bunch of workarounds to bump
up the latency before calling those guys so clearly it won't
end up doing the right thing. Also not sure if the calculations
based on the method1/2 results are safe agaisnt overflows so
it might not work all that well in any case. Let's put the
early check back.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181221171436.8218-3-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>