drm/vmwgfx: Stop messing about with plane->fb/old_fb/crtc

drm/vmwgfx: Stop messing about with plane->fb/old_fb/crtc

plane->fb/old_fb/crtc should no longer be used by atomic
drivers. Stop messing about with them.

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-7-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>