drm/i915: define I915 during i915 driver build

The xe driver will reuse i915 display code by compiling it separately as
part of xe. We'll want to be able to distinguish between building the
i915 display code for i915 and xe. Define I915 when building i915.

Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/97558201836115b91cbe32840239df855d0c2e4c.1694514689.git.jani.nikula@intel.com