drm/msm/dp: add write support in simulation mode

Currently simulation only supports reading data from script
which impersonates a sink device. Further enhancement of script
needs source device to support writing data to it. Add writing
data in simulation so that script can be enhanced for more
simulation support.

Change-Id: I161a3ee30f98d694e80459786edae9dcc4dc91da
CRs-Fixed: 2322388
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Abhijith Desai <desaia@codeaurora.org>