drm/mxsfb: Disable overlay plane in mxsfb_plane_overlay_atomic_disable()

When disabling overlay plane in mxsfb_plane_overlay_atomic_update(),
overlay plane's framebuffer pointer is NULL.  So, dereferencing it would
cause a kernel Oops(NULL pointer dereferencing).  Fix the issue by
disabling overlay plane in mxsfb_plane_overlay_atomic_disable() instead.

Fixes: cb285a53 ("drm: mxsfb: Replace mxsfb_get_fb_paddr() with drm_fb_cma_get_gem_addr()")
Cc: stable@vger.kernel.org # 5.19+
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230612092359.784115-1-victor.liu@nxp.com

drivers/gpu/drm/mxsfb/mxsfb_kms.c

@@ -611,6 +611,14 @@ static void mxsfb_plane_overlay_atomic_update(struct drm_plane *plane,
 	writel(ctrl, mxsfb->base + LCDC_AS_CTRL);
 }
 
+static void mxsfb_plane_overlay_atomic_disable(struct drm_plane *plane,
+					       struct drm_atomic_state *state)
+{
+	struct mxsfb_drm_private *mxsfb = to_mxsfb_drm_private(plane->dev);
+
+	writel(0, mxsfb->base + LCDC_AS_CTRL);
+}
+
 static bool mxsfb_format_mod_supported(struct drm_plane *plane,
 				       uint32_t format,
 				       uint64_t modifier)
@@ -626,6 +634,7 @@ static const struct drm_plane_helper_funcs mxsfb_plane_primary_helper_funcs = {
 static const struct drm_plane_helper_funcs mxsfb_plane_overlay_helper_funcs = {
 	.atomic_check = mxsfb_plane_atomic_check,
 	.atomic_update = mxsfb_plane_overlay_atomic_update,
+	.atomic_disable = mxsfb_plane_overlay_atomic_disable,
 };
 
 static const struct drm_plane_funcs mxsfb_plane_funcs = {