drm/amd: Re-introduce property to control adaptive backlight modulation

drm/amd: Re-introduce property to control adaptive backlight modulation

commit 0887054d14ae ("drm/amd: Drop abm_level property") dropped the
abm level property in favor of sysfs control. Since then there have
been discussions that compositors showed an interest in modifying
a vendor specific property instead.

So re-introduce the abm level property, but with different semantics.
Rather than being an integer it's now an enum. One of the enum options
is 'sysfs', and that is because there is still a sysfs file for use by
userspace when the compositor doesn't support this property.

If usespace has not modified this property, the default value will
be for sysfs to control it. Once userspace has set the property stop
allowing sysfs control.

The property is only attached to non-OLED eDP panels.

Cc: Xaver Hugl <xaver.hugl@kde.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>