drm/msm: enable both dsi drivers together

Create a new config for DSI staging driver.

Conflicts:
	drivers/gpu/drm/msm/Makefile

Change-Id: I76879210bf51c6b30de94166f170ceae5001e1c7
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>
[narendram@codeaurora.org: resolve makefile conflict]
Signed-off-by: Narendra Muppalla <narendram@codeaurora.org>