drm/msm/dsi: Implement qcom, dsi-phy-regulator-ldo-mode for 28nm PHY

drm/msm/dsi: Implement qcom, dsi-phy-regulator-ldo-mode for 28nm PHY

The DSI PHY regulator supports two regulator modes: LDO and DCDC.
This mode can be selected using the "qcom,dsi-phy-regulator-ldo-mode"
device tree property.

However, at the moment only the 20nm PHY driver actually implements
that option. Add a check in the 28nm PHY driver to program the
registers correctly for LDO mode.

Tested-by: Nikita Travkin <nikitos.tr@gmail.com> # l8150
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023165617.28738-1-stephan@gerhold.net