Duplicate FB_BLANK_ constants as BACKLIGHT_POWER__ constants in the
backlight header file. Allows backlight drivers to avoid including
the fbdev header file and removes a compile-time dependency between
the two subsystems.

The new BACKLIGHT_POWER_ constants have the same values as their
FB_BLANK_ counterparts. Hence UAPI and internal semantics do not
change. The backlight drivers can be converted one by one. Each
instance of FB_BLANK_UNBLANK becomes BACKLIGHT_POWER_ON, each of
FB_BLANK_POWERDOWN becomes BACKLIGHT_POWER_OFF, and FB_BLANK_NORMAL
becomes BACKLIGHT_POWER_REDUCED.

Backlight code or drivers do not use FB_BLANK_VSYNC_SUSPEND and
FB_BLANK_HSYNC_SUSPEND, so no new constants for these are being
added.

The semantics of FB_BLANK_NORMAL appear inconsistent. In fbdev,
NORMAL means display off with sync enabled. In backlight code,
this translates to turn the backlight off, but some drivers
interpret it as backlight on. So we keep the current code as is,
but mark BACKLIGHT_POWER_REDUCED as deprecated. Drivers should be
fixed and the constant removed. This affects ams369fg06 and a few
DRM panel drivers.

v2:
- rename BL_CORE_ power constants to BACKLIGHT_POWER_ (Sam)
- fix documentation
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240624152033.25016-2-tzimmermann@suse.deSigned-off-by: Lee Jones <lee@kernel.org>