drm/radeon: Fix oops upon driver load on PowerXpress laptops

drm/radeon: Fix oops upon driver load on PowerXpress laptops

Nicolai Stange reports the following oops which is caused by
dereferencing rdev->pdev before it's subsequently set by
radeon_device_init().  Fix it.

  BUG: unable to handle kernel NULL pointer dereference at 00000000000007cb
  IP: radeon_driver_load_kms+0xeb/0x230 [radeon]
  ...
  Call Trace:
   drm_dev_register+0x146/0x1d0 [drm]
   drm_get_pci_dev+0x9a/0x180 [drm]
   radeon_pci_probe+0xb8/0xe0 [radeon]
   local_pci_probe+0x45/0xa0
   pci_device_probe+0x14f/0x1a0
   driver_probe_device+0x29c/0x450
   __driver_attach+0xdf/0xf0
   ? driver_probe_device+0x450/0x450
   bus_for_each_dev+0x6c/0xc0
   driver_attach+0x1e/0x20
   bus_add_driver+0x170/0x270
   driver_register+0x60/0xe0
   ? 0xffffffffc0508000
   __pci_register_driver+0x4c/0x50
   drm_pci_init+0xeb/0x100 [drm]
   ? vga_switcheroo_register_handler+0x6a/0x90
   ? 0xffffffffc0508000
   radeon_init+0x98/0xb6 [radeon]
   do_one_initcall+0x52/0x1a0
   ? __vunmap+0x81/0xb0
   ? kmem_cache_alloc_trace+0x159/0x1b0
   ? do_init_module+0x27/0x1f8
   do_init_module+0x5f/0x1f8
   load_module+0x27ce/0x2be0
   SYSC_finit_module+0xdf/0x110
   ? SYSC_finit_module+0xdf/0x110
   SyS_finit_module+0xe/0x10
   do_syscall_64+0x67/0x150
   entry_SYSCALL64_slow_path+0x25/0x25

Fixes: 7ffb0ce31cf9 ("drm/radeon: Don't register Thunderbolt eGPU with vga_switcheroo")
Reported-and-tested-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/cfb91ba052af06117137eec0637543a2626a7979.1495135190.git.lukas@wunner.de