amdgpu/pm: prevent array underflow in vega20_odn_edit_dpm_table()

In the PP_OD_EDIT_VDDC_CURVE case the "input_index" variable is capped at
2 but not checked for negative values so it results in an out of bounds
read.  This value comes from the user via sysfs.

Fixes: d5bf2653 ("drm/amd/powerplay: added vega20 overdrive support V3")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>