drm/i915: Inline skl_update_pipe_wm() into its only caller

skl_update_pipe_wm() is quite pointless now. Just inline it into
skl_compute_wm().

v2: s/skl_build_pipe_wm/skl_update_pipe_wm/ in the commit message (Matt)

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-10-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>