drm/amdgpu: remove useless code in RAS

Module parameter amdgpu_ras_mask has been involved in
the calculation of ras support capability, so drop this
redundant code.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>