drm/i915: Remove dead update_wm_pre assignment from SKL wm code

drm/i915: Remove dead update_wm_pre assignment from SKL wm code

SKL+ do not use crtc_state->update_wm_pre, so there is absolutely no
point it setting it. crtc_state->update_wm_pre only exists as a
temporary hack for pre-g4x platforms until we redo their
watermarks to be be atomic.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181113172330.26069-3-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>