drm/i915: don't chnage the original mode in dp_mode_fixup

We should only frob adjusted_mode. This is in preparation of
a massive patch by Laurent Pinchart to make the mode argument
const.

After the previous two prep patches the only thing left is to clean up
things a bit. I've opted to pass in an adjust_mode param to
dp_adjust_dithering because that way we can be sure to avoid
duplicating this logic between mode_valid and mode_fixup - which was
the cause behind a dp link bw calculation bug in the past.

Also mark the mode argument of pch_panel_fitting const.

v2: Split up the mode->clock => adjusted_mode->clock change,
as suggested by Chris Wilson.

Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 2a1625d..7180cc8 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -56,7 +56,7 @@
 void
 intel_pch_panel_fitting(struct drm_device *dev,
 			int fitting_mode,
-			struct drm_display_mode *mode,
+			const struct drm_display_mode *mode,
 			struct drm_display_mode *adjusted_mode)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;