drm/i915: Constify adjusted_mode

Make adjusted_mode const whereever we don't have to modify it. This only
covers cases when we have a local adjusted_mode variable, and doesn't
make any difference for cases where we just dereference
pipe_config->adjusted_mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>