drm/i915: Use adjusted_mode in DSI PLL calculations

adjusted_mode contains our real timings, not requested_mode. Use the
correct thing in DSI PLL code.

Also constify adjusted_mode since we don't change it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>