drm: msm: sde: add api to get if dma wait is needed

In the physical encoder, when the timing engine
is flushed, driver needs to wait for the dma
flush to be done. This change adds support to
get the status from the physical encoder if wait
is needed.

Change-Id: I52442713231fc7e054bbd3c4f472d6fce2e6d73c
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>