drm/i915: Make debugfs/per_file_stats scale better

Currently we walk the entire list of obj->vma for each obj within a file
to find the matching vma of this context. Since we know we are searching
for a particular vma bound to a user context, we can use the rbtree to
search for it rather than repeatedly walk everything.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808162407.28121-1-chris@chris-wilson.co.uk