drm/amdgpu/powerplay: drop a redundant NULL check

Left over from an earlier rev of the patch.

Acked-by: Colin Ian King <colin.king@canonical.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Colin King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>