drm/fb: fix CONFIG_DRM=m && CONFIG_FB=n

The previous commit to move the parsing into the core drm created a
new situation and a soft dependency on the CONFIG_FB. We really don't
want to make this a hard dependency so just wrap the one place that
actually needs an fb symbol.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>