drm/msm/dp: add support for different execution modes

Add support for switching different execution modes like
hardware mode, software mode or both. In hardware mode,
DP works as normal. In software mode, DP driver works on
IO mapped buffers instead of hardware. This mode can be
used to simulate different variants of hardware.

Change-Id: I102b2e37220048d819131bcc4b6d5083499915ee
Signed-off-by: Samantha Tran <samtran@codeaurora.org>
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>