drm/radeon: add SI DPM quirk for Sapphire R9 270 Dual-X 2G GDDR5

Seems to have problems with high mclks.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=76490



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