drm/msm/dpu: fix error return code in dpu_encoder_init

Fix to return negative error code -ENOMEM with the use of
ERR_PTR from dpu_encoder_init.

Fixes: 25fdd593

 ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Chen Tao <chentao107@huawei.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>