drm: ati_pcigart: Fix limit check in drm_ati_pcigart_init().

The variable 'max_pages' is ambiguous.  There are two concepts
of "pages" being used in this function.

First, we have ATI GART pages which are always 4096 bytes.
Then, we have system pages which are of size PAGE_SIZE.

Eliminate the confusion by creating max_ati_pages and
max_real_pages.  Calculate and use them as appropriate.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>