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

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: 52b3ef21e9c1 ("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.com
Reviewed-by: Lyude <lyude@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>