drm/i915: dual link pipe selection for bxt

Pipe is assigned based on the port, but it should be
based on current crtc. Correcting the same in this patch.

v2: Use macro BXT_PIPE_SELECT(pipe) (Daniel)

Signed-off-by: Deepak M <m.deepak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>