drm/amdgpu/display/dce11: only enable FBC when selected

Causes a black screen on a Stoney laptop.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=108577


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