drm/msm/dpu: Add check for pstates

As kzalloc may fail and return NULL pointer,
it should be better to check pstates
in order to avoid the NULL pointer dereference.

Fixes: 25fdd5933e4c ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/514160/
Link: https://lore.kernel.org/r/20221206080236.43687-1-jiasheng@iscas.ac.cn
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
