drm/i915: Split the crtc_mode_set function along HAS_PCH_SPLIT() lines.

This path, which shouldn't be *that* complicated, is now so littered
with per-chipset tweaks that it's hard to trace the order of what
happens.  HAS_PCH_SPLIT() is the most radical change across chipsets,
so it seems like a natural split to simplify the code.

This first commit just copies the existing code without changing
anything.

v2: updated to track removal of call to intel_enable_plane from i9xx_crtc_mode_set
Signed-off-by: Eric Anholt <eric@anholt.net>
Hella-acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.h

@@ -203,6 +203,12 @@ struct drm_i915_display_funcs {
 	int (*get_display_clock_speed)(struct drm_device *dev);
 	int (*get_fifo_size)(struct drm_device *dev, int plane);
 	void (*update_wm)(struct drm_device *dev);
+	int (*crtc_mode_set)(struct drm_crtc *crtc,
+			     struct drm_display_mode *mode,
+			     struct drm_display_mode *adjusted_mode,
+			     int x, int y,
+			     struct drm_framebuffer *old_fb);
+
 	/* clock updates for mode set */
 	/* cursor updates */
 	/* render clock increase/decrease */