drm/i915/display: add intel_display -> drm_device backpointer

As a baby step towards making struct intel_display the main data
structure for display, add a backpointer to struct drm_device that can
be used instead of &i915->drm.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/47dd7bc4aae3c10b18097824e37617c072c66c0b.1713358679.git.jani.nikula@intel.com

drivers/gpu/drm/i915/display/intel_display_core.h

@@ -283,6 +283,9 @@ struct intel_wm {
 };
 
 struct intel_display {
+	/* drm device backpointer */
+	struct drm_device *drm;
+
 	/* Display functions */
 	struct {
 		/* Top level crtc-ish functions */

drivers/gpu/drm/i915/display/intel_display_device.c

@@ -927,6 +927,9 @@ void intel_display_device_probe(struct drm_i915_private *i915)
 	const struct intel_display_device_info *info;
 	u16 ver, rel, step;
 
+	/* Add drm device backpointer as early as possible. */
+	i915->display.drm = &i915->drm;
+
 	if (HAS_GMD_ID(i915))
 		info = probe_gmdid_display(i915, &ver, &rel, &step);
 	else