drm/amdgpu: enable MULTI_MON_PP_MCLK_SWITCH DC feature at default

With this, for multiple monitors in sync(e.g. with the same model),
mclk switching will be allowed. That helps saving some idle power on
some ASICs(e.g. Polaris).

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