drm/vmwgfx: Fix LDU X blank screen until mode change issue

vmw_ldu_crtc_helper_commit() is not called if
drm_atomic_crtc_needs_modeset() decides nothing related to CRTC timing has
changed.

So a better place for this code is in vmw_ldu_primary_plane_atomic_update()
since we will need to update ld->fb every time the FB is updated.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>