drm: rcar-du: Remove local CRTC enabled state

The atomic framework guarantees that the CRTC enable and disable
functions will only be called when needed, there's no need to duplicate
the CRTC state check. By replacing the local CRTC enabled state check at
resume time with the CRTC active state we can remove the local CRTC
enabled state altogether.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>