habanalabs: restructure hl_mmap

Arrange the hl_mmap code to be more structured and expandable for the
future. Add better defines that describe our usage of the vm_pgoff.

Note that I shamelessly took the code and defines from the amdkfd driver
(my previous driver).

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>