disp: msm: dsi: add check to avoid null pointer access

Add a check for valid pointer in the DSI CTRL logging macros
to fix null pointer access issues.

Change-Id: I92576e1db6c2d8b52c2adddd8c964bc2455536e4
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>