drm/i915: Mark skl_update_plane and skl_disable_plane as static

make W=1 caught the implicit prototypes (as would sparse):

drivers/gpu/drm/i915/intel_sprite.c:462:1: error: no previous prototype for ‘skl_update_plane’ [-Werror=missing-prototypes]
 skl_update_plane(struct intel_plane *plane,
 ^~~~~~~~~~~~~~~~
drivers/gpu/drm/i915/intel_sprite.c:487:1: error: no previous prototype for ‘skl_disable_plane’ [-Werror=missing-prototypes]
 skl_disable_plane(struct intel_plane *plane, struct intel_crtc *crtc)
 ^~~~~~~~~~~~~~~~~

Fixes: 1e364f90 ("drm/i915/gen11: Program the Y and UV plane for planar mode correctly, v3.")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024105402.18915-1-chris@chris-wilson.co.uk