drm/i915: add generic __to_intel_display()

Add generic __to_intel_display() macro that accepts either struct
drm_i915_private * or struct intel_display *. This is to be used for
transitional stuff that eventually needs to be converted to use struct
intel_display *, and therefore is not part of to_intel_display().

Add new intel_display_conversion.h to host the helper to avoid
duplication between xe and i915 drivers.

v2: put it in the new header (Rodrigo)
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a999ff8183659a4df68d439ebd31c19b5c56852a.1713358679.git.jani.nikula@intel.com

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

+/* SPDX-License-Identifier: MIT */
+/* Copyright © 2024 Intel Corporation */
+
+/*
+ * This header is for transitional struct intel_display conversion helpers only.
+ */
+
+#ifndef __INTEL_DISPLAY_CONVERSION__
+#define __INTEL_DISPLAY_CONVERSION__
+
+/*
+ * Transitional macro to optionally convert struct drm_i915_private * to struct
+ * intel_display *, also accepting the latter.
+ */
+#define __to_intel_display(p)						\
+	_Generic(p,							\
+		 const struct drm_i915_private *: (&((const struct drm_i915_private *)(p))->display), \
+		 struct drm_i915_private *: (&((struct drm_i915_private *)(p))->display), \
+		 const struct intel_display *: (p),			\
+		 struct intel_display *: (p))
+
+#endif /* __INTEL_DISPLAY_CONVERSION__ */