loader/dri3: fix swap out of order when changing swap interval

This fixes SPECVIEWPERF13 creo test case hang:

1. Client: send present pixmap request (serial=1) when swap_interval==1 and increase send_sbc=1
2. Server: pend the request before vblank arrives
3. Client: set swap_interval=0 (so set XCB_PRESENT_OPTION_ASYNC), send another present pixmap request (serial=2), increase send_sbc=2
4. Server: handle the async request immediately and send complete event (serial=2)
5. Client: handle the event and set recv_sbc=event->serial=2
6. Server: vblank arrives so handle pending request and send complete event (serial=1)
7. Client: handle the event and set recv_sbc=event->serial=1
8. Client: someone call loader_dri3_swapbuffer_barrier() and waiting on recv_sbc==send_sbc, but no one will set recv_sbc=2 again

So basically it's caused by swap happens out of order. This commit fixes the problem by waiting on the pending sync swaps all done when switching to async mode, so move 6&7 before 3.

Attach the xtrace when problem happens:

  005:<:003e: 72: Present-Request(148,1): Pixmap window=0x03000002 pixmap=0x0300000b serial=1 valid=0x00000000 update=0x00000000 x_off=0 y_off=0 target_crtc=0x00000000 wait_fence=0x00000000 idle_fence=0x0300000c options=0 target_msc=4294967296 divisor=0 remainder=0 notifies=;
  ...
  005:<:0041: 72: Present-Request(148,1): Pixmap window=0x03000002 pixmap=0x03000011 serial=2 valid=0x00000000 update=0x00000000 x_off=0 y_off=0 target_crtc=0x00000000 wait_fence=0x00000000 idle_fence=0x03000012 options=Async target_msc=0 divisor=0 remainder=0 notifies=;
  005:>:0041: Event Generic(35) Present(148) IdleNotify(2) event=0x03000006 window=0x03000002 serial=2 pixmap=0x03000011 idle_fence=0x03000012
  005:>:0041: Event Generic(35) Present(148) CompleteNotify(1) kind=Pixmap(0x00) mode=Copy(0x00) event=0x03000006 window=0x03000002 serial=2 ust=7505462213117739011 msc=3565046193979392
  005:>:0041: Event Generic(35) Present(148) IdleNotify(2) event=0x03000006 window=0x03000002 serial=1 pixmap=0x0300000b idle_fence=0x0300000c
  005:>:0041: Event Generic(35) Present(148) CompleteNotify(1) kind=Pixmap(0x00) mode=Copy(0x00) event=0x03000006 window=0x03000002 serial=1 ust=7505533793042694147 msc=3565050488946688

CC: @mareko @daenzer