drm/msm/gem: Simplify vmap vs LRU tracking

vmap'ing is just pinning in disguise.  So treat it as such and simplify
the LRU tracking.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/527837/
Link: https://lore.kernel.org/r/20230320144356.803762-6-robdclark@gmail.com