drm/vc4: Remove conflicting framebuffers before callind bind_all

The bind hooks will modify their controller registers, so simplefb is
going to be unusable anyway. Let's avoid any transient state where it
could still be in the system but no longer functionnal.

Acked-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215095117.176435-4-maxime@cerno.tech