drm/i915: Clean up skl+ vs. icl+ watermark computation

Make a cleaner split between the skl+ and icl+ ways of computing
watermarks. This way skl_build_pipe_wm() doesn't have to know any
of the gritty details of icl+ master/slave planes.

We can also simplify a bunch of the lower level code by pulling
the plane visibility checks a bit higher up.

v2: WARN_ON(!visible) for the icl+ master plane case (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181127165726.31122-1-ville.syrjala@linux.intel.com