drm/msm/gem: Consolidate pin/unpin paths

Avoid having multiple spots where we increment/decrement pin_count (and
associated LRU updating)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496130/
Link: https://lore.kernel.org/r/20220802155152.1727594-8-robdclark@gmail.com