drm/amdgpu: correct initial cp_hqd_quantum for gfx9

commit 9f52c25f

 upstream.

didn't read the value of mmCP_HQD_QUANTUM from correct
register offset

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>