drm: msm: sde: last command support for reg dma

For each control path last command needs to be sent to reg dma hardware
engine to de-assert it's busy signal which indicates that current frame
can be displayed. Change adds support to send the last command before
the control is flushed to ensure.

Change-Id: I8219d0130bad5e9032a791e357a284ae55bcc852
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>