drm: fix issue with handle lookup for a 0 handle

On 32-bit PPC a 0 handle is valid for AGP space, the 32/64 lookup
doesn't handle 0 correctly.

From: Ben Herrenschmidt <benh@kernel.crashing.org> and Paul Mackerras <paulus@samba.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>
1 file changed