drm: fix _DRM_GEM addmap error message

Fix the error message: this is add, not rm.
Move the closing brace to proper spot: _DRM_GEM branch should not be
included in the block.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>