drm/i915: Make skl_write_{plane,cursor}_wm() static

Someone forgot to make skl_write_{plane,cursor}_wm() static when
removing the prototypes from the header. Sparse isn't pleased.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lyude <cpaul@redhat.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Fixes: e62929b3 ("drm/i915/gen9+: Program watermarks as a separate step during evasion, v3.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479846113-24745-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Lyude <lyude@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -3851,7 +3851,7 @@ static void skl_write_wm_level(struct drm_i915_private *dev_priv,
 	I915_WRITE(reg, val);
 }
 
-void skl_write_plane_wm(struct intel_crtc *intel_crtc,
+static void skl_write_plane_wm(struct intel_crtc *intel_crtc,
 			       const struct skl_plane_wm *wm,
 			       const struct skl_ddb_allocation *ddb,
 			       int plane)
@@ -3875,7 +3875,7 @@ void skl_write_plane_wm(struct intel_crtc *intel_crtc,
 			    &ddb->y_plane[pipe][plane]);
 }
 
-void skl_write_cursor_wm(struct intel_crtc *intel_crtc,
+static void skl_write_cursor_wm(struct intel_crtc *intel_crtc,
 				const struct skl_plane_wm *wm,
 				const struct skl_ddb_allocation *ddb)
 {