drm/radeon: drop pitcairn dpm quirks

No longer necessary with the new 58 mc ucode.

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



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