drm: Remove command line guard for universal planes

Universal planes are ready to leave 'experimental' state so drop the
kernel command line parameter that we've been hiding them behind.
Userspace clients that wish to receive universal planes will still need
to opt-in by setting the appropriate capability bit, so this should have
no impact on existing userspace.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>