drm/msm: Lazily create context VM

In the next commit, a way for userspace to opt-in to userspace managed
VM is added.  For this to work, we need to defer creation of the VM
until it is needed.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661490/