drm/radeon/kms: fix memory leak in radeon_driver_load_kms

This patch fixes following kmemleak report:
unreferenced object 0xffff88022cb53000 (size 4096):
  comm "work_for_cpu", pid 97, jiffies 4294672345
  backtrace:
    [<ffffffff810eb222>] create_object+0x19f/0x2a0
    [<ffffffff810eb422>] kmemleak_alloc+0x26/0x4c
    [<ffffffff810e363f>] __kmalloc+0x187/0x1b0
    [<ffffffffa005f3db>] kzalloc.clone.0+0x13/0x15 [radeon]
    [<ffffffffa005f403>] radeon_driver_load_kms+0x26/0xe1 [radeon]
    [<ffffffffa0017432>] drm_get_dev+0x37f/0x480 [drm]
    [<ffffffffa007f424>] radeon_pci_probe+0x15/0x269 [radeon]
    [<ffffffff811f8779>] local_pci_probe+0x17/0x1b
    [<ffffffff8105ffbb>] do_work_for_cpu+0x18/0x2a
    [<ffffffff81063c38>] kthread+0x8a/0x92
    [<ffffffff81012cba>] child_rip+0xa/0x20
    [<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>