drm/amd/pm: fix the Out-of-bounds read warning

using index i - 1U may beyond element index
for mc_data[] when i = 0.

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>