drm/doc: Add a label for the PRIME Buffer Sharing chapter

So that it can be referenced from e.g. DOC comments.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/drm-mm.rst

@@ -395,6 +395,8 @@ VMA Offset Manager
 .. kernel-doc:: drivers/gpu/drm/drm_vma_manager.c
    :export:
 
+.. _prime_buffer_sharing:
+
 PRIME Buffer Sharing
 ====================