mesa/main: do not allow etc2 enums on gles1

mesa/main: do not allow etc2 enums on gles1

ctx->Extensions.ARB_ES3_compatibility is set regardless of the API
that's used, so checking for those direcly will always enable
extensions when they are supported by the driver.

But there's no extension enabling ETC2 for OpenGL ES 1.x, so we
shouldn't allow those enums there.

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