drm/i915: Unexport is_semiplanar_uv_plane()

This function is only used by intel_fb.c, so unexport it.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020195138.1841242-6-imre.deak@intel.com