drm/amd/powerplay: delete some dead code

The "size" variable is unsigned.  We never pass invalid sizes to this
function and we already used it as an array offset earlier so it's
too late to check here.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>