x86: use sparse_memory_present_with_active_regions() on UMA

There's no need to use call memory_present() manually on UMA because
initmem_init() sets up early_node_map by calling
e820_register_active_regions().

[ Impact: cleanup ]

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1241699742.17846.31.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>