habanalabs: fix possible hole in device va

cb_map_mem() uses gen_pool_alloc() to get virtual address for
mapping a CB.
The mapping is done in chunks of page size, so if the CB size is
larger, it is possible that the allocated virtual addresses won't
be consecutive.
User retrieves this device VA which returns the virtual address
in the first va_block. If there is a "hole" in the virtual addresses,
user can configure a HW block with a bad device VA.

Signed-off-by: farah kassabri <fkassabri@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>