disp: msm: dsi: Add missing LAST_COMMAND support in debugfs path

Existing debugfs path for sending DCS command never sets LAST_COMMAND
flag due to which commands are not sent to panel. This patches fixes
the same.

Change-Id: I5fd4cbfca2842736a28d3e75e74f49dafa83e4cc
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>