drm/i915: Move skip_intermediate_wm handling into ilk_compute_intermediate_wm()

No point in cluttering the common codepaths with the
skip_intermediate_wm handling. Just move it into
ilk_compute_intermediate_wm() as those are the only
platforms using this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181108151013.24064-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc