disp: msm: sde: add check to fix null pointer dereference

In pstate_cmp the plane_state structures are accessed
without checking it is allocated or not, which leads to
null pointer dereference. To fix it NULL check is added.

Change-Id: I5982138b396b70979205d87bc9aa260d9501fee0
Signed-off-by: Mahadevan <mahap@codeaurora.org>