drm/bochs: Drop best_encoder

This is the default for atomic drivers.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190221155857.19773-2-daniel.vetter@ffwll.ch
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>