drm/amd/display: Add VESA SCR case for default aux backlight

[How & Why]
When determining default aux backlight level, read from
DPCD address 0x734 for VESA SCR on OLED.

Reviewed-by: Felipe Clark <felipe.clark@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Iswara Nagulendran <iswara.nagulendran@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>