drm/msm/dsi_phy_7nm: implement PHY disabling

Implement phy_disable() callback to disable DSI PHY lanes and blocks
when phy is not used.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: 1ef7c99d ("drm/msm/dsi: add support for 7nm DSI PHY/PLL")
Signed-off-by: Rob Clark <robdclark@chromium.org>