drm/amd/powerplay: fix a reversed condition

This test was reversed so it would end up leading to a NULL dereference.

Fixes: 4630f0fa ('drm/amd/powerplay: add Carrizo smu support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>