drm/xe: Include build directory

When doing out-of-tree builds with O= or KBUILD_OUTPUT=, it's important
to also add the directory where the target is saved. Otherwise any file
generated by the build system may not be available for other targets
depending on it.

The $(obj) is added automatically when building the entire kernel,
but it's not added when M=drivers/gpu/drm/xe is added.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230526164358.86393-12-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>