drm/ast: fix the ast open key function

When porting from UMS I mistyped this from the wrong place, AST noticed
and pointed it out, so we should fix it to be like the X.org driver.

Reported-by: Y.C. Chen <yc_chen@aspeedtech.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>