OMAP: DSS2: DSI: Disable interface when disabling the display

Once the DSI PLL is separated from the DSI display a full DSI reset will
not be performed every time the display is enabled so the interface and
VCs must be disabled when disabling the display. If the VCs are not
disabled some register accesses will abort.

Signed-off-by: Ville Syrjälä <ville.syrjala@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>