drm/amd/display: Remove unused macro from dcn21

[Why]
SOC_BOUNDING_BOX_VALID is unused and not required for dcn21.

[How]
Remove it.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>