backlight: Nuke unused backlight.props.state states

The backlight power state handling is supremely confusing. We have:
- props.power, using FB_BLANK_* defines
- props.fb_blank, using the same, but deprecated int favour of
  props.state
- props.state, using the BL_CORE_* defines
- and finally a bunch of backlight drivers treat brightness == 0 as
  off. But of course not all of them.

This is way too much confusion to fix in a simple patch, but at least
prevent more hilarity from spreading by removing the unused BL_CORE_*
defines. I have no idea why exactly anyone would need that.

Wrt the ideal state, we really just want a boolean state. The 4 power
saving states that the fbdev subsystem uses are overkill in todays hw
(this was only relevant for VGA and similar analog circuits like
TV-out), the new drm atomic modeset api simplified even the uapi to a
simple bool. And there was never a valid technical reason to have the
intermediate fbdev power states for backlights (those really only can
be either off or on).

Cleanup motivated by Meghana's questions about all this.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>