Revert "drm/amd/display: set backlight level limit to 1"

This reverts commit 0cafc82f.

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>