drm/drm_plane.h: fix grammar of the comment

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230409131547.494128-1-15330273260@189.cn


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>