drm: use ida to allocate connector ids

This makes it so that reloading a module does not cause all the
connector ids to change, which are user-visible and sometimes used
for configuration.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>