Upon the initialization of a user context, map the host memory page of
the virtual MSI-X doorbell in the device MMU.
A reserved VA is used for this purpose, so user can use it directly
without any allocation/map operation.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>