drm/msm/sde: add check for interrupt while disabling auto-refresh

When cont-splash feature is enabled, we were adding delay
when auto-refresh is disabled so that any pending
frames will be transferred before attaching SMMU.
Instead of adding usleep_range(), add check for Ping-Pong
done interrupt when auto-refresh is disabled.
Add support for polling the read-pointer since we don't
have interrupt handler enabled during probe sequence.

Change-Id: I08f807b321eb6980aea881d4885d38f29bf7789d
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>