drm/amd/display: Fix brightness level not retained over reboot

[Why]
During boot up and resume the DC layer will reset the panel
brightness to fix a flicker issue.

It will cause the dm->actual_brightness is not the current panel
brightness level. (the dm->brightness is the correct panel level)

[How]
Set the backlight level after do the set mode.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Fixes: d9e86582 ("drm/amd/display: Simplify brightness initialization")
Reported-by: Mark Herbert <mark.herbert42@gmail.com>
Closes: drm/amd#3655


Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 7875afaf)
Cc: stable@vger.kernel.org