drm/amdgpu: Fix use of interruptible waiting

1. The signal interrupt can affect the expected behaviour.
2. There is no good mechanism to handle the corresponding error.
When signal interrupt happens, unpin is not called.
As a result, inside AMDGPU, the statistic of pin size will be wrong.

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>