drm/msm/gem: Consolidate shrinker trace

Combine separate trace events for purge vs evict into one.  When we add
support for purging/evicting active buffers we'll just add more info
into this one trace event, rather than adding a bunch more events.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496133/
Link: https://lore.kernel.org/r/20220802155152.1727594-13-robdclark@gmail.com