habanalabs/gaudi: move mmu_prepare to context init

Currently mmu_prepare is located at context switch.
Since we support a single context, no reason to reconfigure
the MMU registers every context switch.

Signed-off-by: Ofir Bitton <obitton@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>