drm/amd/powerplay: Fix Vega10 power profile switching

Clock index 0 is a valid index that is needed to restore the default
graphics power profile. Use ~0 to indicate a failure to find a clock
index. This fixes the clocks getting stuck in the compute power
profile after running a compute application on Vega10.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>