drm/ingenic: Fix incorrect assumption about plane->index

plane->index is NOT the index of the color plane in a YUV frame.
Actually, a YUV frame is represented by a single drm_plane, even though
it contains three Y, U, V planes.

v2-v3: No change

Cc: stable@vger.kernel.org # v5.3
Fixes: 90b86fcc ("DRM: Add KMS driver for the Ingenic JZ47xx SoCs")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-1-paul@crapouillou.net