drm/xe: Fix wa_oob codegen recipe for external module builds

drm/xe: Fix wa_oob codegen recipe for external module builds

When building with 'make M=drivers/gpu/drm/xe modules', kbuild invokes
scripts/Makefile.build with obj=., causing $(obj) to expand to '.'.
Make normalizes './xe_gen_wa_oob' to 'xe_gen_wa_oob' when constructing
the $^ automatic variable (target name normalization), so the recipe
command becomes just 'xe_gen_wa_oob ...' without any path prefix, and
the shell cannot find the tool.

Fix by replacing $^ with explicit $(obj)/xe_gen_wa_oob and
$(src)/<rules-file> references in both wa_oob recipe commands.
In recipe strings, make does not apply target name normalization, so
$(obj)/xe_gen_wa_oob correctly expands to './xe_gen_wa_oob' and the
shell can execute it. This matches the pattern already used by other
DRM drivers (e.g. radeon's mkregtable).

Fixes: f037e0b78e6d ("drm/xe: add xe_device_wa infrastructure")
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-xe@lists.freedesktop.org
Assisted-by: GitHub_Copilot:claude-sonnet-4.6
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/20260604074501.172129-1-thomas.hellstrom@linux.intel.com