drm/amdgpu: add codes to clear AccVGPR for arcturus

AccVGPRs are newly added in arcturus. Before reading these
registers, they should be initialized. Otherwise edc error
happens, when RAS is enabled.

v2: reuse the existing logical to calculate register size
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>