drm/amd/display: Add VC for VESA Aux Backlight Control

[WHY]
There is no way to distinguish
the static backlight control type
being used and the VABC support
without the use of a debugger or
reading DPCD registers.

[HOW]
Add Visual Confirm support
for VESA Aux-based Backlight Control.

Reviewed-by: Harry Vanzylldejong <harry.vanzylldejong@amd.com>
Signed-off-by: Iswara Nagulendran <Iswara.Nagulendran@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>