drm/amd/display: free bo used for dmub bounding box

fix a memleak introduced by not removing the buffer object for use with
early dmub bounding box value storage

Fixes: 234e9455 ("drm/amd/display: Enable copying of bounding box data from VBIOS DMUB")
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>