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

drivers/gpu/drm/i915/Kconfig.debug

@@ -18,6 +18,7 @@ config DRM_I915_WERROR
 config DRM_I915_DEBUG
         bool "Enable additional driver debugging"
         depends on DRM_I915
+        select DEBUG_FS
         select PREEMPT_COUNT
         select I2C_CHARDEV
         select DRM_DP_AUX_CHARDEV