drm/i915: introduce dp_to_i915() helper

No functional change. But let's get first i915 pointer
directly from intel_dp so we can clean up a lot of code
later.
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827223021.7145-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -1313,6 +1313,12 @@ dp_to_lspcon(struct intel_dp *intel_dp)
 	return &dp_to_dig_port(intel_dp)->lspcon;
 }
 
+static inline struct drm_i915_private *
+dp_to_i915(struct intel_dp *intel_dp)
+{
+	return to_i915(dp_to_dig_port(intel_dp)->base.base.dev);
+}
+
 static inline struct intel_digital_port *
 hdmi_to_dig_port(struct intel_hdmi *intel_hdmi)
 {