drm/i915: Switch eviction code to use vmas

drm/i915: Switch eviction code to use vmas

The execbuf wants to do relocations usings vmas, so we need a
vma->exec_list. The eviction code also uses the old obj execbuf list
for it's own book-keeping, but would really prefer to deal in vmas
only. So switch it over to the new list.

Again this is just a prep patch for the big execbuf vma conversion.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Split out from Ben's big execbuf vma patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>