drm/i915: replace DRM_DEBUG_SELFTEST with DRM_KUNIT_TEST

drm/i915: replace DRM_DEBUG_SELFTEST with DRM_KUNIT_TEST

DRM_DEBUG_SELFTEST was removed in commit fc8d29e298cf (drm: selftest:
convert drm_mm selftest to KUnit) and all functions under it were
converted to KUnit, under the DRM_KUNIT_TEST option

This conversion however did not occur in the Kconfig.debug file in the
i915 directory.

This patch replaces the select for DRM_DEBUG_SELFTEST, an option that no
longer exists, with the correct select, DRM_KUNIT_TEST.

Signed-off-by: Ruben Wauters <rubenru09@aol.com>
Link: https://lore.kernel.org/r/20250701115511.5445-1-rubenru09@aol.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>