drm: ati_pcigart: simplify page_count manipulations

From: Nick Piggin <npiggin@suse.de>

Allocate a compound page for the user mapping instead of tweaking the page
refcounts.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>