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 <[e-mail]>
Signed-off-by: Philipp Zabel <[e-mail]>

drivers/gpu/drm/imx/ipuv3-plane.c

···
 	kfree(ipu_plane);
 }
 
-void ipu_plane_state_reset(struct drm_plane *plane)
+static void ipu_plane_state_reset(struct drm_plane *plane)
 {
 	struct ipu_plane_state *ipu_state;
 
···
 	plane->state = &ipu_state->base;
 }
 
-struct drm_plane_state *ipu_plane_duplicate_state(struct drm_plane *plane)
+static struct drm_plane_state *
+ipu_plane_duplicate_state(struct drm_plane *plane)
 {
 	struct ipu_plane_state *state;
 
···
 	return &state->base;
 }
 
-void ipu_plane_destroy_state(struct drm_plane *plane,
-			     struct drm_plane_state *state)
+static void ipu_plane_destroy_state(struct drm_plane *plane,
+				    struct drm_plane_state *state)
 {
 	struct ipu_plane_state *ipu_state = to_ipu_plane_state(state);