drm/i915: Select DRM_VGEM for igt

igt/prime_vgem (and others) depends upon VGEM so automatically select it when
enabling i915 debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1468059777-10205-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/Kconfig.debug

@@ -20,6 +20,7 @@ config DRM_I915_DEBUG
         depends on DRM_I915
         select PREEMPT_COUNT
         select X86_MSR # used by igt/pm_rpm
+        select DRM_VGEM # used by igt/prime_vgem (dmabuf interop checks)
         default n
         help
           Choose this option to turn on extra driver debugging that may affect