DPLL is not needed for DSI

v2: Rebase due to added DSI PLL assertion patch.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NShobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>