disp: msm: sde: add checks to avoid null pointer dereference

This change adds required checks to avoid accessing the
null pointer in case where memory allocation fails while
dumping the registers in memory.

Change-Id: Ic087a1bd041d193820289672d325a78795c48c14
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>