drm/msm/dpu: drop dpu_encoder_phys_ops::destroy

Drop the dpu_encoder_phys_ops' destroy() callback. No phys backend
implements it anymore, so it is useless.

Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570058/
Link: https://lore.kernel.org/r/20231201211845.1026967-13-dmitry.baryshkov@linaro.org