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

Cc: Deepak Rawat <drawat@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-5-ville.syrjala@linux.intel.comReviewed-by: Sinclair Yeh <syeh@vmware.com>