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.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>