x86/gart: Rewrite early_gart_iommu_check() comment

... to actually explain what the function is trying to do.

Reported-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <x86@kernel.org>
Link: http://lkml.kernel.org/r/20181101155314.30690-1-bp@alien8.de