drm/amdgpu/display: restore the backlight on modeset (v2)

To stay consistent with the user's setting.

v2: rebase on multi-eDP support

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1337


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>