drm: msm: dsi: fix potential NULL pointer dereference

Add checker to confirm the pointer is not NULL first, to avoid
NULL pointer dereference happens.

Change-Id: Id9060725037be235303bbc0a82d91d3d2008245c
CRs-Fixed: 2202957
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>