compositor-drm: no need to clear inherited_mode

The head was just zalloc()'d, there is no need to memset it to zero.

If a function fails, it is preferable it leaves the output arguments
untouched.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>