intel: Remove gratuitous assert on bo_reference.

This couldn't be triggered except by overflow, since there's an assert
in unreference to catch the usual failure of over-unreferencing.

intel/intel_bufmgr_gem.c

@@ -351,7 +351,6 @@ drm_intel_gem_bo_reference(drm_intel_bo *bo)
 {
 	drm_intel_bo_gem *bo_gem = (drm_intel_bo_gem *) bo;
 
-	assert(atomic_read(&bo_gem->refcount) > 0);
 	atomic_inc(&bo_gem->refcount);
 }