drm/msm/disp: check the return value of kzalloc()

kzalloc() is a memory allocation function which can return NULL when
some internal memory errors happen. So it is better to check it to
prevent potential wrong memory access.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/tencent_B3E19486FF39415098B572B7397C2936C309@qq.com
Signed-off-by: Rob Clark <robdclark@chromium.org>