drm/radeon: Add early unregister of firmware fb's

Without this, we attempt the handover too late, the firmware fb
might be accessing the chip simultaneously to us re-initializing
various parts of it, which might frighten babies or cause all sort
of nasty psychologic trauma to kitten.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[danvet: add cc: stable, forward ported and compile-fixed for X86]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[airlied: move to even earlier in module load.]
Signed-off-by: Dave Airlie <airlied@redhat.com>