drm/radeon: add a force flush to delay work when radeon

In the Linux kernel, the following vulnerability has been resolved:

drm/radeon: add a force flush to delay work when radeon

Although radeon card fence and wait for gpu to finish processing current batch rings,

there is still a corner case that radeon lockup work queue may not be fully flushed,

and meanwhile the radeon_suspend_kms() function has called pci_set_power_state() to

put device in D3hot state.

Per PCI spec rev 4.0 on 5.3.1.4.1 D3hot State.

> Configuration and Message requests are the only TLPs accepted by a Function in

> the D3hot state. All other received Requests must be handled as Unsupported Requests,

> and all received Completions may optionally be handled as Unexpected Completions.

This issue will happen in following logs:

Unable to handle kernel paging request at virtual address 00008800e0008010

CPU 0 kworker/0:3(131): Oops 0

pc = [<ffffffff811bea5c>] ra = [<ffffffff81240844>] ps = 0000 Tainted: G W

pc is at si_gpu_check_soft_reset+0x3c/0x240

ra is at si_dma_is_lockup+0x34/0xd0

v0 = 0000000000000000 t0 = fff08800e0008010 t1 = 0000000000010000

t2 = 0000000000008010 t3 = fff00007e3c00000 t4 = fff00007e3c00258

t5 = 000000000000ffff t6 = 0000000000000001 t7 = fff00007ef078000

s0 = fff00007e3c016e8 s1 = fff00007e3c00000 s2 = fff00007e3c00018

s3 = fff00007e3c00000 s4 = fff00007fff59d80 s5 = 0000000000000000

s6 = fff00007ef07bd98

a0 = fff00007e3c00000 a1 = fff00007e3c016e8 a2 = 0000000000000008

a3 = 0000000000000001 a4 = 8f5c28f5c28f5c29 a5 = ffffffff810f4338

t8 = 0000000000000275 t9 = ffffffff809b66f8 t10 = ff6769c5d964b800

t11= 000000000000b886 pv = ffffffff811bea20 at = 0000000000000000

gp = ffffffff81d89690 sp = 00000000aa814126

Disabling lock debugging due to kernel taint

Trace:

[<ffffffff81240844>] si_dma_is_lockup+0x34/0xd0

[<ffffffff81119610>] radeon_fence_check_lockup+0xd0/0x290

[<ffffffff80977010>] process_one_work+0x280/0x550

[<ffffffff80977350>] worker_thread+0x70/0x7c0

[<ffffffff80977410>] worker_thread+0x130/0x7c0

[<ffffffff80982040>] kthread+0x200/0x210

[<ffffffff809772e0>] worker_thread+0x0/0x7c0

[<ffffffff80981f8c>] kthread+0x14c/0x210

[<ffffffff80911658>] ret_from_kernel_thread+0x18/0x20

[<ffffffff80981e40>] kthread+0x0/0x210

Code: ad3e0008 43f0074a ad7e0018 ad9e0020 8c3001e8 40230101

<88210000> 4821ed21

So force lockup work queue flush to fix this problem.