drm/amd/display: Fix compile error with ACPI disabled

The fallback code for getting default backlight caps was using
the wrong variable name.  Fix it.

Fixes: https://lists.freedesktop.org/archives/dri-devel/2018-November/197752.html


Signed-off-by: David Francis <David.Francis@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>