drm: Allow range of 0 for drm_mm_insert_node_in_range()

We gracefully handle the caller specifying a zero range, so don't force
them to special case that condition if it naturally falls out of their
setup. What we don't check is if the end < start, so keep that as an
assert for an illegal call.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626094330.3556-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_mm.c

@@ -472,7 +472,7 @@ int drm_mm_insert_node_in_range(struct drm_mm * const mm,
 	u64 remainder_mask;
 	bool once;
 
-	DRM_MM_BUG_ON(range_start >= range_end);
+	DRM_MM_BUG_ON(range_start > range_end);
 
 	if (unlikely(size == 0 || range_end - range_start < size))
 		return -ENOSPC;