drm: Make DRM_DEBUG_MM depend on STACKTRACE_SUPPORT

0day continues to complain about trying to save a stacktrace for the
users of the drm_mm range allocator. This time, it is that m68k has no
save_stack_trace(), which is apparently guarded by STACKTRACE_SUPPORT.
Make it depend so!

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161109143906.11057-1-chris@chris-wilson.co.uk

drivers/gpu/drm/Kconfig

@@ -37,6 +37,7 @@ config DRM_DEBUG_MM
 	bool "Insert extra checks and debug info into the DRM range managers"
 	default n
 	depends on DRM=y
+	depends on STACKTRACE_SUPPORT
 	select STACKDEPOT
 	help
 	  Enable allocation tracking of memory manager and leak detection on