drm/i2c: tda998x: split tda998x_encoder_dpms into enable/disable

This fits better with the drm_bridge callbacks for when this
driver becomes a drm_bridge.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
[edited by rmk to just split the tda998x_encoder_dpms() function
 and restore the double-disable protection we originally had,
 preserving original behaviour.]
Tested-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>