drm/i915: Dump pipe config when intel_modeset_pipe_config fails.

This makes it easier to debug issues like https://bugs.freedesktop.org/show_bug.cgi?id=93477

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Chris Wilson <|chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/843f4327-1574-cf8e-0776-adbb0d58c2c0@mblankhorst.nl
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>