gpu: nvgpu: add platform specific get_iova_addr()

gpu: nvgpu: add platform specific get_iova_addr()

Add platform specific API pointer (*get_iova_addr)()
which can be used to get iova/physical address from
given scatterlist and flags

Use this API with g->ops.mm.get_iova_addr() instead
of calling API gk20a_mm_iova_addr() which makes it
platform specific

Bug 1605653

Change-Id: I798763db1501bd0b16e84daab68f6093a83caac2
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/713089
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>