This reverts commit 0cafc82fae.
This breaks some apps that assume 0 is minimum brightness.
Revert for 4.20. This is fixed properly for drm-next/4.21 in:
"drm/amd: Don't fail on backlight = 0"
However, that patch depends on more extensive changes to the
backlight interface which are too invasive for -fixes.
Fixes: Bugzilla: https://bugs.freedesktop.org/108668
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
63088da947