x86: gart: Clean up the code a bit

Clean up various small stylistic details in the GART code. No
functionality changed.

Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: muli@il.ibm.com
Cc: joerg.roedel@amd.com
LKML-Reference: <1258287594-8777-2-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>