drm/amdgpu: Add flag to wipe VRAM on release

This memory allocation flag will be used to indicate BOs containing
sensitive data that should not be leaked to other processes.

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