drm/msm: dpu: Fix "WARNING: invalid free of devm_ allocated data"

'dpu_enc' is a member of 'drm_enc'
And 'drm_enc' got allocated with devm_kzalloc in dpu_encoder_init.

This gives this error message:
./drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c:459:1-6:
 WARNING: invalid free of devm_ allocated data

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>