drm/amd/pm: Fix negative array index read

Avoid using the negative values
for clk_idex as an index into an array pptable->DpmDescriptor.

V2: fix clk_index return check (Tim Huang)

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>