drm/msm/gem: Remove "valid" tracking

This was a small optimization for pre-soft-pin userspace.  But mesa
switched to soft-pin nearly 5yrs ago.  So lets drop the optimization
and simplify the code.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/568328/