drm/amdgpu: Allocate root PD on correct partition

file_priv needs to be setup firstly, otherwise, root PD
will always be allocated on partition 0, even if opening
the device from other partitions.

Fixes: ffc6deb7 ("drm/amdkfd: Store xcp partition id to amdgpu bo")
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>