drm/msm: get an iova from the address space instead of an id

In the future we won't have a fixed set of addresses spaces.
Instead of going through the effort of assigning a ID for each
address space just use the address space itself as a token for
getting / putting an iova.

This forces a few changes in the gem object however: instead
of using a simple index into a list of domains, we need to
maintain a list of them. Luckily the list will be pretty small;
even with dynamic address spaces we wouldn't ever see more than
two or three.

CRs-Fixed: 2050484
Change-Id: Ic0dedbad4495f02a21135217f3605b93f8b8dfea
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>