drm/amd/pm: workaround for audio noise issue

On some Intel platforms, audio noise can be detected due to
high pcie speed switch latency.
This patch leaverages ppfeaturemask to fix to the highest pcie
speed then disable pcie switching.

v2:
coding style fix

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>