drm/amdgpu: add debug flag to enable RAS ACA

Use debug_mask=0x10 (BIT.4) param to help enable RAS ACA.
(RAS ACA is disabled by default.)

Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>