contigmalloc1 panic on install

[mark tinguely]

>  Regarding this issue, I've got a 5.1-CURRENT snapshot (as of 7-9) which is
>  still displaying this problem.  I have a debug kernel and can provide some
>  more detail :
>
>  agp0: <Ali Generic host to PCI bridge> mem 0xf8000000-0xfbffffff at device
>  0.0 on pci0
>  agp0: allocatnig GATT for aperture size 0M
>  panic: contigmalloc1: size must not be 0
>  Debugger("panic")
>  Stopped at Debugger+0x54: xchgl %ebx,in_Debugger.0

there is really two problems (in my opinion):

	1) AGP_GET_APERTURE() is failing to return the aperture.
	2) agp_alloc_gatt() should return an error (0) if AGP_GET_APERTURE()
	   fails instead of malloc/contigmalloc. This would result in no
	   monitor use, but that would be better than a panic.

--Mark.