drm/i915: Pass intel_crtc to intel_disable_pipe() and intel_wait_for_pipe_off()

Just pass the intel_crtc around instead of dev_priv+pipe.

Also make intel_wait_for_pipe_off() static since it's only used in
intel_display.c.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>