drm/amd/pm: correct the thermal alert temperature limit settings

Do the maths in celsius degree. This can fix the issues caused
by the changes below:

drm/amd/pm: correct Vega20 swctf limit setting
drm/amd/pm: correct Vega12 swctf limit setting
drm/amd/pm: correct Vega10 swctf limit setting

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>