drm/msm/hdmi: use devres helper for runtime PM management

Use devm_pm_runtime_enable() to enable runtime PM. This way its effect
will be reverted on device unbind/destruction.

Fixes: 6ed9ed48

 ("drm/msm/hdmi: Set up runtime PM for HDMI")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/499647/
Link: https://lore.kernel.org/r/20220826093927.851597-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>