drm/amdkfd: fix KFDSVMRangeTest.PartialUnmapSysMemTest fails

drm/amdkfd: fix KFDSVMRangeTest.PartialUnmapSysMemTest fails

[ RUN      ] KFDSVMRangeTest.PartialUnmapSysMemTest
/home/yifan/brahma/libhsakmt/tests/kfdtest/src/KFDTestUtil.cpp:245: Failure
Value of: (hsaKmtAllocMemory(m_Node, m_Size, m_Flags, &m_pBuf))
  Actual: 1
Expected: HSAKMT_STATUS_SUCCESS
Which is: 0
/home/yifan/brahma/libhsakmt/tests/kfdtest/src/KFDTestUtil.cpp:248: Failure
Value of: (hsaKmtMapMemoryToGPUNodes(m_pBuf, m_Size, __null, mapFlags, 1, &m_Node))
  Actual: 1
Expected: HSAKMT_STATUS_SUCCESS
Which is: 0
/home/yifan/brahma/libhsakmt/tests/kfdtest/src/KFDTestUtil.cpp:306: Failure
Expected: ((void *)__null) != (ptr), actual: NULL vs NULL
Segmentation fault (core dumped)
[          ] Profile: Full Test
[          ] HW capabilities: 0x9

kernel log:

[  102.029150]  ret_from_fork+0x22/0x30
[  102.029158] ---[ end trace 15c34e782714f9a3 ]---
[ 3613.603598] amdgpu: Address: 0x7f7149ccc000 already allocated by SVM
[ 3613.610620] show_signal_msg: 27 callbacks suppressed

These is race with deferred actions from previous memory map
changes (e.g. munmap).Flush pending deffered work to avoid such case.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>