drm/virtio: Stop updating plane->crtc

We want to get rid of plane->crtc on atomic drivers. Stop setting it.

v2: s/fb/crtc/ in the commit message (Gerd)

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-12-ville.syrjala@linux.intel.com


Reviewed-by: Sinclair Yeh <syeh@vmware.com>