drm/arm: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/arm/malidp_planes.c:49:25: warning: no previous prototype for 'malidp_duplicate_plane_state' [-Wmissing-prototypes]
drivers/gpu/drm/arm/malidp_planes.c:66:6: warning: no previous prototype for 'malidp_destroy_plane_state' [-Wmissing-prototypes]

In fact, both functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>

drivers/gpu/drm/arm/malidp_planes.c

@@ -58,7 +58,8 @@ static void malidp_de_plane_destroy(struct drm_plane *plane)
 	devm_kfree(plane->dev->dev, mp);
 }
 
-struct drm_plane_state *malidp_duplicate_plane_state(struct drm_plane *plane)
+static struct
+drm_plane_state *malidp_duplicate_plane_state(struct drm_plane *plane)
 {
 	struct malidp_plane_state *state, *m_state;
 
@@ -77,8 +78,8 @@ struct drm_plane_state *malidp_duplicate_plane_state(struct drm_plane *plane)
 	return &state->base;
 }
 
-void malidp_destroy_plane_state(struct drm_plane *plane,
-				struct drm_plane_state *state)
+static void malidp_destroy_plane_state(struct drm_plane *plane,
+				       struct drm_plane_state *state)
 {
 	struct malidp_plane_state *m_state = to_malidp_plane_state(state);