drm/amdkfd: Cleanup KFD nodes creation

kfd node allocation outside kfd->num_nodes loop is not needed and causes
memory leak because kfd->num_nodes is at least equal to 1.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>