Don't use Maxmem when the amount of memory is meant. Use realmem instead.
Maxmem is not only a MD variable, it represents the highest physical memory
address in use. On systems where memory is sparsely layed-out the highest
memory address and the amount of memory are not interchangeable. Scaling the
AGP aperture based on the actual amount of memory (= realmem) rather than
the available memory (= physmem) makes sure there's consistent behaviour
across architectures.
agp_i810.c:
While arguably the use of Maxmem can be considered correct, replace its use
with realmem anyway. agp_i810.c is specific to amd64, i386 & pc98, which
have a dense physical memory layout. Avoiding Maxmem here is done with an
eye on copy-n-paste behaviour in general and to avoid confusion caused by
using realmem in agp.c and Maxmem in agp_i810.c.
In both cases, remove the inclusion of md_var.h
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