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

Change struct allocation of encoder's phys backend data to use
drmm_kzalloc(). 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/570051/
Link: https://lore.kernel.org/r/20231201211845.1026967-12-dmitry.baryshkov@linaro.org