drm/amdkfd: Fix a bug when calculating save_area_used_size

workgroup context data writes from m->cp_hqd_cntl_stack_size, so we
should deduct it when calculating the used size.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>