radeon_pm: fix oops in hwmon_attributes_visible() and radeon_hwmon_show_temp_thresh()

Since commit ec39f64b ("drm/radeon/dpm: Convert to use
devm_hwmon_register_with_groups") radeon_hwmon_init() is using
hwmon_device_register_with_groups(), which sets `rdev' as a device
private driver_data, while hwmon_attributes_visible() and
radeon_hwmon_show_temp_thresh() are still waiting for `drm_device'.

Fix them by using dev_get_drvdata(), in order to avoid this oops:

  BUG: unable to handle kernel paging request at 0000000000001e28
  IP: [<ffffffffa02ae8b4>] hwmon_attributes_visible+0x18/0x3d [radeon]
  PGD 15057e067 PUD 151a8e067 PMD 0
  Oops: 0000 [#1] PREEMPT SMP
  Call Trace:
    internal_create_group+0x114/0x1d9
    sysfs_create_group+0xe/0x10
    sysfs_create_groups+0x22/0x5f
    device_add+0x34f/0x501
    device_register+0x15/0x18
    hwmon_device_register_with_groups+0xb5/0xed
    radeon_hwmon_init+0x56/0x7c [radeon]
    radeon_pm_init+0x134/0x7e5 [radeon]
    radeon_modeset_init+0x75f/0x8ed [radeon]
    radeon_driver_load_kms+0xc6/0x187 [radeon]
    drm_dev_register+0xf9/0x1b4 [drm]
    drm_get_pci_dev+0x98/0x129 [drm]
    radeon_pci_probe+0xa3/0xac [radeon]
    pci_device_probe+0x6e/0xcf
    driver_probe_device+0x98/0x1c4
    __driver_attach+0x5c/0x7e
    bus_for_each_dev+0x7b/0x85
    driver_attach+0x19/0x1b
    bus_add_driver+0x104/0x1ce
    driver_register+0x89/0xc5
    __pci_register_driver+0x58/0x5b
    drm_pci_init+0x86/0xea [drm]
    radeon_init+0x97/0x1000 [radeon]
    do_one_initcall+0x7f/0x117
    load_module+0x1583/0x1bb4
    SyS_init_module+0xa0/0xaf

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>