x86, 32-bit: clean up find_low_pfn_range()

Impact: cleanup

Split find_low_pfn_range() into two functions:

 - lowmem_pfn_init()
 - highmem_pfn_init()

The former gets called if all of RAM fits into lowmem,
otherwise we call highmem_pfn_init().

Signed-off-by: Ingo Molnar <mingo@elte.hu>