[media] atomisp: de-duplicate sh_css_mmu_set_page_table_base_index

Between the ISP2400 and ISP2401 code base this function moved file. The merge
of the drivers left us with two version in ifdefs. Resolve this down to a
single copy.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>