Provide proper aliases to enable/disable MFE.

The numeric values are ambiguous and misleading (e.g: user may misunderstand
setting mfmode to 1 is to enable MFE but actually it is to disable MFE, and
set it to be 5 or above is meaningless).

MFX_MF_MANUAL hasn't been exposed since it is to be implemented.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>