drm/msm/dsi: Enable runtime PM

Some devices power the DSI PHY/PLL through a power rail that we model
as a GENPD. Enable runtime PM to make it suspendable.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/543352/
Link: https://lore.kernel.org/r/20230620

-topic-dsiphy_rpm-v2-2-a11a751f34f0@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>