drm/amd/display: Fix memory leak in dm_set_writeback()

'wb_info' needs to be freed on error paths or it would leak the memory.

Smatch pointed this out.

Fixes: c81e13b9 ("drm/amd/display: Hande writeback request from userspace")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>