drm/msm/sde: add null checks to sde kms prepare/commit

This patch adds null checks for kms and dev pointers before
attempting to dereference them.

CRs-Fixed: 2037667
Change-Id: Id3517c8c33155df6d47ddd019731d039201e9fa6
Signed-off-by: Clarence Ip <cip@codeaurora.org>