drm/radeon/kms: fix use of vram scratch page on evergreen/ni

This hunk seems to have gotten lost when I rebased the patch.

Reported-by: Sylvain Bertrand <sylvain.bertrand@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>