drm/stm: ltdc: Remove unused function plane_to_ltdc

drm/stm: ltdc: Remove unused function plane_to_ltdc

The function are defined in the ltdc.c file, but not called
anywhere, so delete the unused function.

drivers/gpu/drm/stm/ltdc.c:494:35: warning: unused function 'encoder_to_ltdc'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9403
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624024113.54850-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>