drmOpenByName: remove redundant drmAvailable check

drmOpenByName() is a static function that is only called by drmOpen().
drmOpen() already checks drmAvailable(), so the check in
drmOpenByName() is redundant.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>