drm/dsi: set last command flag for generic DCS commands

Currently DRM DSI framework does not set the last command
property while transferring commands to DSI host driver,
so DSI host driver does not trigger the command in HW.
This change fixes the same by setting last command property
for all generic DCS commands in framework.

Change-Id: I101ae2fa61970f4d5f9259e884cb883dfb5b66a3
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>