drm/amdgpu: Fix use of interruptible waiting

If amdgpu_bo_reserve function is interrupted by signal,
amdgpu_bo_kunmap function is not called.

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>