drm/radeon: disable mclk dpm on R7 260X

Setting higher mclks seems to cause stability issues
on some R7 260X boards.  Disable it for now for stability
until we find a proper fix.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=75992



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