drm/radeon: free iio for atombios when driver shutdown

Fix below kmemleak when unload radeon driver:

unreferenced object 0xffff9f8608ede200 (size 512):
  comm "systemd-udevd", pid 326, jiffies 4294682822 (age 716.338s)
  hex dump (first 32 bytes):
    00 00 00 00 c4 aa ec aa 14 ab 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<0000000062fadebe>] kmem_cache_alloc_trace+0x2f1/0x500
    [<00000000b6883cea>] atom_parse+0x117/0x230 [radeon]
    [<00000000158c23fd>] radeon_atombios_init+0xab/0x170 [radeon]
    [<00000000683f672e>] si_init+0x57/0x750 [radeon]
    [<00000000566cc31f>] radeon_device_init+0x559/0x9c0 [radeon]
    [<0000000046efabb3>] radeon_driver_load_kms+0xc1/0x1a0 [radeon]
    [<00000000b5155064>] drm_dev_register+0xdd/0x1d0
    [<0000000045fec835>] radeon_pci_probe+0xbd/0x100 [radeon]
    [<00000000e69ecca3>] pci_device_probe+0xe1/0x160
    [<0000000019484b76>] really_probe.part.0+0xc1/0x2c0
    [<000000003f2649da>] __driver_probe_device+0x96/0x130
    [<00000000231c5bb1>] driver_probe_device+0x24/0xf0
    [<0000000000a42377>] __driver_attach+0x77/0x190
    [<00000000d7574da6>] bus_for_each_dev+0x7f/0xd0
    [<00000000633166d2>] driver_attach+0x1e/0x30
    [<00000000313b05b8>] bus_add_driver+0x12c/0x1e0

iio is allocated in atom_index_iio() called by atom_parse(),
but it doesn't got released when the dirver is shutdown.
Fix this kmemleak by free it in radeon_atombios_fini().

Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>