drm/amdkfd: Fix memory leakage

This patch fixes potential memory leakage and seg fault
in  _gpuvm_import_dmabuf() function

Fixes: d4ec4bdc ("drm/amdkfd: Allow access for mmapping KFD BOs")
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>