drm/msm/gem: Split out inuse helper

Prep for a following patch, where it gets a bit more complicated.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20220411215849.297838-5-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>