drm/amd/display: Write default Vesa Aux backlight control in dmub

drm/amd/display: Write default Vesa Aux backlight control in dmub

[WHY]
Some OLED panels require driver to write Aux BL before link training or
turning on backlight; otherwise monitor brightness will change.

[HOW]
Write the default Vesa Aux backlight control in dmub.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Derek Lai <Derek.Lai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>