i945: improve get_top_of_ram()

i945: improve get_top_of_ram()

The current version doesn't honor TSEG, and fails to
report the correct top of RAM if IGD is disabled. This
is because it uses the BSM (base of stolen RAM) register.
In that case, we should use the TOLUD register.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Acked-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6483 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1