configure: drop explicit DRI_DIRS assignment on some platforms/arches

Both x86_64|amd64 and *bsd, already set the full range of available
classic dri drivers. Drop the explicit assignment, and fall back to
the generic default.
Keep explicit list from plafroms/arches that do not handle the default
list.

Update help strings, to explicitly mention "classic" for applicable
DRI drivers.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
1 file changed