alpha/drm: Cleanup Alpha support in DRM generic code

Remove an obsolete Alpha adjustment, and modify another,
to go with the current Alpha architecture support.

Signed-off-by: Jay Estabrook <jay.estabrook@gmail.com>
Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>