drm/msm/sde: modify flush api in control path

This change removes the get_bitmask_<hw_blk> api in control path.
Instead of getting the bitmask and updating the pending bitmask callers
can directly update the pending bitmask with a single api usage.
Additionally updated other flush api's to make them ready
for future hw versions.

Change-Id: I2ad414ad18028f44d7f058c02cd1b2989432be9c
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>