drm/arm/hdlcd: Reject atomic commits that disable only the plane

The HDLCD engine needs an active plane while the CRTC is active, as
it will start scanning out data from HDLCD_REG_FB_BASE once it gets
enabled. Make sure that the only available plane doesn't get disabled
while the CRTC remains active, as this will scanout invalid data.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>