drm/radeon/dpm: reduce r7xx vblank mclk threshold to 200

Most laptops seems to have a vblank period of less than
300 and mclk switching works fine.  Drop the quirk and
set the default threshold to 200.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=70701



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>