Defer filling enum-type mixers information

Currently, all information about all mixers are fetched during mixer_open().
For enum mixers, an additional step gets the string for each enumerated value -
this can lead to a lot of ioctls and take time even when these mixers aren't
used afterward.

Defer fetching integer/string mapping values when needed (when getting/setting
a enum mixer).

Change-Id: I3681699eb55631a2bc7a1e66bf08aff3a845534f
Signed-off-by: David Coutherut <davidx.coutherut@intel.com>
Signed-off-by: David Wagner <david.wagner@intel.com>
Author-Tracking-BZ: 113043