drm/i915: Select DEBUG_FS for our test suite

Ensure that we build our test kernels with DEBUG_FS enabled as many of
our igt tests require poking around debugfs/dri.

This should also fixup the kbuild complaint that we tried to select
SW_SYNC without meeting its dependencies.

Reported-by:  <kbuild-all@01.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171012092147.28986-1-chris@chris-wilson.co.uk