i915: split some DP modesetting code into a separate function

this is just prep work for mst support.

Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index fa19744..40086e1 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -718,6 +718,7 @@
 void intel_ddi_get_config(struct intel_encoder *encoder,
 			  struct intel_crtc_config *pipe_config);
 
+void intel_ddi_init_dp_buf_reg(struct intel_encoder *encoder);
 
 /* intel_display.c */
 const char *intel_output_name(int output);