drm/amdgpu: clear UVD VCPU buffer when err_event_athub generated

The err_event_athub error will mess up the buffer and cause UVD resume hang.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <hawking.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>