drm/fb: add support for not enabling fbcon on non-desktop displays [v2]

We don't want fbcon to get used on non-desktop dislays,
don't pass them as enabled connectors to the fb helper setup.

This prevents my HMD from getting disorted fbcon, and from
affecting other displays console.

v2: Change description from non-standard to non-desktop

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>