drm/radeon: check alloc_apertures() success in radeon_kick_out_firmware_fb()

Check for alloc_apertures() memory allocation failure, and propagate an
error code in case the allocation failed.

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>