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: 879b3b65 ("drm/fb_dma: Add generic get_scanout_buffer() for drm_panic")
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240426121121.241366-1-jfalempe@redhat.com