drm: require a non_NULL drm_crtc.primary

If a CRTC is missing a legacy primary plane pointer, a lot of things
will be broken for user-space: fbdev stops working and the entire legacy
uAPI stops working.

Require all drivers to populate drm_crtc.primary to prevent these
issues. Warn if it's NULL.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211184634.74534-3-contact@emersion.fr