drm/msm: Add $debugfs/gem stats on resident objects

Currently nearly everything, other than newly allocated objects which
are not yet backed by pages, is pinned and resident in RAM.  But it will
be nice to have some stats on what is unpinned once that is supported.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210405174532.1441497-6-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>