mesa/main: clean up S3_s3tc check

mesa/main: clean up S3_s3tc check

S3_s3tc is the extension that enables this functionality on desktop, so
let's check for that one. The _mesa_has_S3_s3tc() helper already
verifies the API according to the extension-table.

As for the second hunk, we currently already only expose
EXT_texture_compression_s3tc on desktop so by using the helper instead,
we get rid of this detail here, and once we enable it for GLES we'll
automaticall get the interaction right.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>