drm/radeon: Move fbdev cleanup code into fb_destroy callback

Fbdev calls struct fb_ops.fb_destroy after cleaning up the final
reference to an fbdev framebuffer. Move radeon's fbdev cleanup code
there.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>