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/108668Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1524,13 +1524,6 @@ static int amdgpu_dm_backlight_update_status(struct backlight_device *bd)
 {
 	struct amdgpu_display_manager *dm = bl_get_data(bd);
 
-	/*
-	 * PWM interperts 0 as 100% rather than 0% because of HW
-	 * limitation for level 0.So limiting minimum brightness level
-	 * to 1.
-	 */
-	if (bd->props.brightness < 1)
-		return 1;
 	if (dc_link_set_backlight_level(dm->backlight_link,
 			bd->props.brightness, 0, 0))
 		return 0;