drm/msm/dpu: use drmm-managed allocation for dpu_encoder_virt

drm/msm/dpu: use drmm-managed allocation for dpu_encoder_virt

It is incorrect to use devm-managed memory allocations for DRM data
structures exposed to userspace. They should use drmm_ allocations.
Change struct dpu_encoder allocation to use drmm_encoder_alloc(). This
removes the need to perform any actions on encoder destruction.

Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570053/
Link: https://lore.kernel.org/r/20231201211845.1026967-14-dmitry.baryshkov@linaro.org