drm/amd/pm: increase mclk switch threshold to 200 us

To avoid underflow seen on Polaris10 with some 3440x1440
144Hz displays. As the threshold of 190 us cuts too close
to minVBlankTime of 192 us.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>