drm/msm/gem: Split out submit_unpin_objects() helper

Untangle unpinning from unlock/unref loop.  The unpin only happens in
error paths so it is easier to decouple from the normal unlock path.

Since we never have an intermediate state where a subset of buffers
are pinned (ie. we never bail out of the pin or unpin loops) we can
replace the bo state flag bit with a global flag in the submit.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/568335/