drm/i915: fix EDID/sink-based bpp clamping

Since this is run in the compute config stage we need to check
the new_ pointers, i.e the stage output routing, not the current
modeset layout. Also there was a little logic bug in properly skipping
connectors: The old code did not skip any unused connectors and so
clamped to whatever was left in there (usually 0 if that connector
hasn't seen a EDID 1.4 screen ever since boot-up).

This has been broken when moving the pipe bpp selection in

commit 4e53c2e0
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 27 00:44:58 2013 +0100

    drm/i915: precompute pipe bpp before touching the hw

To avoid too much casting switch from drm_ to intel_ types.

Also add a bit of debug output to help reconstructing what's going
on.

v2: Try to clarify this a bit:
- s/pipe_config_set_bpp/compute_baseline_pipe_bpp/ to make it clearer
  at which stage this function is run. Also add a comment about what
  it does.
- Extract the sink clamping into it's own function.

v3: Actually make it compile.

v4: Split out all the prep refactoring to make the bugfix stick out
really badly. Also elaborate a bit in the commit message about the
nature of the bugfix.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7728,8 +7728,8 @@ compute_baseline_pipe_bpp(struct intel_crtc *crtc,
 	/* Clamp display bpp to EDID value */
 	list_for_each_entry(connector, &dev->mode_config.connector_list,
 			    base.head) {
-		if (connector->base.encoder &&
-		    connector->base.encoder->crtc != crtc)
+		if (!connector->new_encoder ||
+		    connector->new_encoder->new_crtc != crtc)
 			continue;
 
 		connected_sink_compute_bpp(connector, pipe_config);