drm/i915: Remove redundant DSI PLL enabling

DSI PLL will get configured during crtc_enable using ->pre_pll_enable
and no need to do in ->mode_set

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>