drm/xe/display: remove compat i915_gem.h

It's only there for GEM_BUG_ON(), but the display code no longer uses
it. Good riddance.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8de7815d9c80125b86e0b85a1af80d4f8686eef6.1712345787.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/xe/compat-i915-headers/i915_drv.h

@@ -19,7 +19,6 @@
 #include "xe_bo.h"
 #include "xe_pm.h"
 #include "xe_step.h"
-#include "i915_gem.h"
 #include "i915_gem_stolen.h"
 #include "i915_gpu_error.h"
 #include "i915_reg_defs.h"

drivers/gpu/drm/xe/compat-i915-headers/i915_gem.h

-/* SPDX-License-Identifier: MIT */
-/*
- * Copyright © 2023 Intel Corporation
- */
-
-#ifndef __I915_GEM_H__
-#define __I915_GEM_H__
-#define GEM_BUG_ON
-#endif