habanalabs: restore vm_pgoff after mmap

Due to using dma_mmap_coherent() to perform mmap of dma memory, we
had to clear the vm_pgoff field before calling that function.

However, that broke the userspace (profiler tool) as they relied
on searching the /proc/self/maps for these values to correctly
"disassemble" the topology recipe.

To re-enable that functionality, the driver can simply restore the
value of vm_pgoff before returning to userspace but after calling
dma_mmap_coherent().

Signed-off-by: Oded Gabbay <ogabbay@kernel.org>