st/dri: move fence implemention into separate file

st/dri: move fence implemention into separate file

Since the fence implementation is not dri2.c specific, put
it in a separate file. This way SW implementations can use this
extension too.

v2: Don't depend on dri2.c for extensions (Emil)
v3: Make this patch only move extension into a separate file (Chad).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>