drm/vmwgfx: Add and connect connector helper function

These helpers won't be called until we flip on the atomic support
flag or set drm_crtc_funcs->set_config to using the atomic
helper.

v2
Use drm_atomic_helper_best_encoder() rather than a vmwgfx-specific one

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>