drm/sun4i: remove simplefb at probe

If simplefb was setup by our bootloader and enabled in the DT, we will have
a first framebuffer loaded in our system.

However, as soon as our DRM driver will load, it will reset the controller,
initialise it and, if the framebuffer emulation is enabled, register a
second framebuffer device.

This is obviously pretty bad, since the first framebuffer will be some kind
of a black hole, with memory still reserved that we can write to safely,
but not displayed anywhere.

Make sure we remove that framebuffer when we probe so we don't end up in
that situation.

Fixes: 9026e0d1 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>