drm/msm: add kms->wait_flush()

First step in re-working the atomic related internal API to prepare for
async updates pending.. ->wait_flush() is intended to block until there
is no in-progress flush.

A crtc_mask is used, rather than an atomic state object, as this will
later be used for async flush after the atomic state is destroyed.

This replaces ->wait_for_crtc_commit_done()

v2: update for review comments

Signed-off-by: Rob Clark <robdclark@chromium.org>