drm/amd/powerplay: add dpm force multiple levels on cz/tonga/fiji/polaris (v2)

Allows you to force multiple levels rather than just one via the new
sysfs interrface.

v2: squash in:
drm/amd/powerplay: ensure clock level set by user is valid.
From Rex.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>