drm/msm/gem: Check for active in shrinker path

Currently in our shrinker path we shouldn't be encountering anything
that is active, but this will change in subsequent patches.  So check
if there are unsignaled fences.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496117/
Link: https://lore.kernel.org/r/20220802155152.1727594-5-robdclark@gmail.com