mesa/st: always use DXT5 when transcoding ASTC format
This fixes artifacts seen in games when using ASTC transcoding, we need to use DXT5 for proper alpha channel support.
Number of components is a block specific property, there is no easy way to see if we will require >1bit alpha support or not, so simply use DXT5 to have support in place.
Signed-off-by: Tapani Pälli tapani.palli@intel.com