drm/xe: eradicate -Ddrm_i915_gem_object=xe_bo

We've now completely stopped using drm_i915_gem_object in display code
that gets built for xe. Kill off the -Ddrm_i915_gem_object=xe_bo
hack. Good riddance.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0eae2b62b635acafe5dc97dc4b205aaa34ce1e53.1726589119.git.jani.nikula@intel.com

drivers/gpu/drm/xe/Makefile

@@ -148,7 +148,6 @@ subdir-ccflags-$(CONFIG_DRM_XE_DISPLAY) += \
 	-I$(src)/display/ext \
 	-I$(src)/compat-i915-headers \
 	-I$(srctree)/drivers/gpu/drm/i915/display/ \
-	-Ddrm_i915_gem_object=xe_bo \
 	-Ddrm_i915_private=xe_device
 
 # Rule to build SOC code shared with i915