[Why]
Backlight is conceptually a property of links, not streams.
All backlight programming is done on links, but there is a
stream property bl_pwm_level that is used to restore backlight
on dpms on and s3 resume.  This is unnecessary, as backlight
is already restored by hardware with no driver intervention.

[How]
Remove bl_pwm_level, and the stream argument to set_backlight

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=109375Signed-off-by: NDavid Francis <David.Francis@amd.com>
Reviewed-by: NHarry Wentland <Harry.Wentland@amd.com>
Acked-by: NAnthony Koo <Anthony.Koo@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 923fe495)