drm/stm: remove conflicting framebuffers

In case of using simplefb or another conflicting framebuffer,
call drm_aperture_remove_framebuffers() to remove memory allocated.

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206134735.13537-1-yannick.fertre@foss.st.com