drm/amdgpu/gmc10: remove dummy read workaround for newer chips

Sienna Cichlid and newer have a hw fix so no longer require
the workaround.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>