drm/msm: Make enable_eviction flag static

No need for it to be visible outside of this one src file.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/491219/
Link: https://lore.kernel.org/r/20220625225454.81039-3-robdclark@gmail.com