drm/amdgpu: fix the coverage issue to clear ArcVPGRs

Set ComputePGMRSRC1.VGPRS as 0x3f to clear all ArcVGPRs.

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>