backend/drm: use DRM_IOCTL_MODE_CLOSEFB instead of RMFB
RMFB implicitly performs a modeset to turn off any CRTC which is using the FB. This prevents seamless transitions between two DRM masters from working.
Use the new CLOSEFB IOCTL which doesn't turn off anything and leave it up to the compositor to turn off outputs on shutdown if it wants to.