x86: iommu: use symbolic constants, not hardcoded numbers

Move symbolic constants into gart.h, and use them instead of hardcoded
constant.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>