drm/amdgpu: fix xclk freq on CHIP_STONEY

According to Alex, most APUs from that time seem to have the same issue
(vbios says 48Mhz, actual is 100Mhz).  I only have a CHIP_STONEY so I
limit the fixup to CHIP_STONEY

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org