drm/i915: Convert open-coded use of vma_pages()

drm/i915: Convert open-coded use of vma_pages()

If we want to know how many pages a VMA spans, we can use vma_pages() to
find out. We have one such invocation inside our faulthandler, so
convert it. (We have two other that want the size in bytes rather than
pages, food for future thought.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161011090656.29554-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>