drm/fb_dma: Add checks in drm_fb_dma_get_scanout_buffer()

plane->state and plane->state->fb can be NULL, so add a check before
dereferencing them.
Found by testing with the imx driver.

Fixes: 879b3b6511fe ("drm/fb_dma: Add generic get_scanout_buffer() for drm_panic")
Signed-off-by: Jocelyn Falempe <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 986c12d8c9a677c094c37bd6aa636b4d4c5ccd46)
Signed-off-by: Maarten Lankhorst <[e-mail]>

drivers/gpu/drm/drm_fb_dma_helper.c

···
 	struct drm_gem_dma_object *dma_obj;
 	struct drm_framebuffer *fb;
 
+	if (!plane->state || !plane->state->fb)
+		return -EINVAL;
+
 	fb = plane->state->fb;
 	/* Only support linear modifier */
 	if (fb->modifier != DRM_FORMAT_MOD_LINEAR)