mesa/main: fix validation of transform-feedback overflow queries

mesa/main: fix validation of transform-feedback overflow queries

ctx->Extensions.ARB_transform_feedback_overflow_query is set based on
the driver-capabilities, not based on the context type. We need to
check against _mesa_has_RB_transform_feedback_overflow_query(ctx)
instead to figure out if the extension is really supported.

This turns usage of GL_TRANSFORM_FEEDBACK_STREAM_OVERFLOW and
GL_TRANSFORM_FEEDBACK_OVERFLOW into errors on ES 3, as required by the
spec.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>