drm/i915/gt: use correct selftest config symbol

drm/i915/gt: use correct selftest config symbol

intel_engine_user.c checks CONFIG_DRM_I915_SELFTESTS before running
the engine UABI isolation check. Kconfig defines DRM_I915_SELFTEST,
without the trailing "S", and the rest of i915 uses
CONFIG_DRM_I915_SELFTEST.

Because CONFIG_DRM_I915_SELFTESTS is not backed by any Kconfig symbol,
the IS_ENABLED() test is always false. Use the existing selftest symbol
so the debug/selftest guarded path can be reached when selftests are
enabled.

This is a source-level fix. It does not claim dynamic hardware
reproduction; the evidence is the Kconfig definition and the inconsistent
guard in intel_engine_user.c.

Fixes: 750e76b4f9f6 ("drm/i915/gt: Move the [class][inst] lookup for engines onto the GT")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260705080225.436-1-pengpeng@iscas.ac.cn
(cherry picked from commit 14a2012a490258f3f93857bc4f1b203405964be7)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>