drm/amd/display: Read back max backlight value at boot
[Why]
If there is no program explicitly setting the backlight
brightness (for example, during a minimal install of linux), the
hardware defaults to maximum brightness but the backlight_device
defaults to 0 value. Thus, settings displays the wrong brightness
value.
[How]
When creating the backlight device, set brightness to max
Signed-off-by: David Francis <David.Francis@amd.com> Reviewed-by: Harry Wentland <Harry.Wentland@amd.com> Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>