drm/i915/panel: Always record the backlight level again (but cleverly)

The commit 47356eb6 introduced a
mechanism to record the backlight level only at disabling time, but it
also introduced a regression.  Since intel_lvds_enable() may be called
without disabling (e.g. intel_lvds_commit() calls it unconditionally),
the backlight gets back to the last recorded value.  For example, this
happens when you dim the backlight, close the lid and open the lid,
then the backlight suddenly goes to the brightest.

This patch fixes the bug by recording the backlight level always
when changed via intel_panel_set_backlight().  And,
intel_panel_{enable|disable}_backlight() call the internal function not
to update the recorded level wrongly.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>