drm/i915: Move buffer pinning and ring selection to intel_crtc_page_flip()

All of the .queue_flip() callbacks duplicate the same code to pin the
buffers and calculate the gtt_offset. Move that code to
intel_crtc_page_flip(). In order to do that we must also move the ring
selection logic there.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>