drm/msm/mdp5: Add check for kzalloc

drm/msm/mdp5: Add check for kzalloc

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

Fixes: 1cff7440a86e ("drm/msm: Convert to using __drm_atomic_helper_crtc_reset() for reset.")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/514154/
Link: https://lore.kernel.org/r/20221206074819.18134-1-jiasheng@iscas.ac.cn
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>