drm/amd/powerplay: remove redundant assignment to variables HiSidd and LoSidd

The variables HiSidd and LoSidd are being initialized with values that
are never read and are being updated a little later with a new value.
The initialization is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>