drm/imx: ipuv3-plane: Make functions static when possible

ipu_plane_state_reset(), ipu_plane_duplicate_state() and
ipu_plane_destroy_state() are only used in this file, so make them static.

This fixes the following sparse warnings:

drivers/gpu/drm/imx/ipuv3-plane.c:275:6: warning: symbol 'ipu_plane_state_reset' was not declared. Should it be static?
drivers/gpu/drm/imx/ipuv3-plane.c:295:24: warning: symbol 'ipu_plane_duplicate_state' was not declared. Should it be static?
drivers/gpu/drm/imx/ipuv3-plane.c:309:6: warning: symbol 'ipu_plane_destroy_state' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>