drm/imx: imx-ldb: reset display clock input when disabling LVDS

The LDB driver changes the attached display interface's input clock mux
to the LDB_DI clock reference. Change it back again when disabling the
LVDS display. Changing back to the PLL5 for example allows to configure
the same DI for HDMI output later.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>