drm/amd/display: Set default brightness according to ACPI

Currently, amdgpu will always set up the brightness at 100% when it
loads.  However this is jarring when the BIOS has it previously
programmed to a much lower value.

The ACPI ATIF method includes two members for "ac_level" and "dc_level".
These represent the default values that should be used if the system is
brought up in AC and DC respectively.

Use these values to set up the default brightness when the backlight
device is registered.

v2: squash in ACPI fix

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>