drm/amd/display: fix default brightness

[Why]
To avoid flickering during boot default brightness level set by BIOS
should be maintained for as much of the boot as feasible.
commit 2fe87f5 ("drm/amd/display: Set default brightness according
to ACPI") attempted to set the right levels for AC vs DC, but brightness
still got reset to maximum level in initialization code for
setup_backlight_device().

[How]
Remove the hardcoded initialization in setup_backlight_device() and
instead program brightness value to match BIOS (AC or DC).  This avoids a
brightness flicker from kernel changing the value.  Userspace may however
still change it during boot.

Fixes: 2fe87f5 ("drm/amd/display: Set default brightness according to ACPI")
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 0747acf)
Cc: stable@vger.kernel.org

Author: superm1

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

@@ -4911,6 +4911,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	dm->backlight_dev[aconnector->bl_idx] =
 		backlight_device_register(bl_name, aconnector->base.kdev, dm,
 					  &amdgpu_dm_backlight_ops, &props);
+	dm->brightness[aconnector->bl_idx] = props.brightness;
 
 	if (IS_ERR(dm->backlight_dev[aconnector->bl_idx])) {
 		DRM_ERROR("DM: Backlight registration failed!\n");
@@ -4978,7 +4979,6 @@ static void setup_backlight_device(struct amdgpu_display_manager *dm,
 	aconnector->bl_idx = bl_idx;
 
 	amdgpu_dm_update_backlight_caps(dm, bl_idx);
-	dm->brightness[bl_idx] = AMDGPU_MAX_BL_LEVEL;
 	dm->backlight_link[bl_idx] = link;
 	dm->num_of_edps++;