drm/i915: Add SW_SYNC to our recommend testing Kconfig

drm/i915: Add SW_SYNC to our recommend testing Kconfig

Since we do use the SW_SYNC in igt for validating dma-fence and
sync_file, and wish to expand usage to cover driver independent portions
of syncobj interaction, ensure SW_SYNC is included in our testing
Kconfig.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170810094036.4307-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>