drm/i915: clean up and simplify i9xx_crtc_mode_set wrt PLL handling

Flat out skip anything to do with PLL if we have a DSI encoder (and thus
DSI PLL). Also skip PLL computation if the encoder has already set
clocks. This allows for some tidying up of the code, including a
superfluous call to intel_limit() for LVDS downclock path.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>